A single word in Arabic may have many apparently irrelevant meanings and yet all be correct.
There are a couple of things you need to understand:
1- Arabic roots as a concept in themselves.
2- Islamic terminology and the role of the Quran in redefining words or adding additional meanings to words.
1. Roots: Arabic roots denote usually one or two central concepts, and words that are derived from these roots often contain many shades of meaning that revolve around these central concepts. Sometimes they might mean something completely different. Sounds very complex and involved, I know, but that's what makes Arabic extremely expressive. When dealing with Classical Arabic, a single word very often cannot be expressed in a single English word. Like for example "Islam", which I explained in your previous question:
What does the word "Islam" mean?
There is a conceptual philosophy behind every word. It is not simply that a word "means" this or that, but on studying Classical Arabic one realizes "why" it means this or that, and how words are interlinked and how their derivation is evolved around concepts, and how therefore it can also mean many other things. Hence when you understand the central concepts that a root might deal with - then you begin to see the common thread that binds seemingly irrelevant words together, regardless of how semantically distant they look when translated into another language.
2. Words that have a different meaning in Islamic terminology from their ordinary usage. This is actually true of any field of knowledge. Usually field-specific jargon gives ordinary words a new meaning. As for Islam, Arabic and Islam are very interlinked. You can never fully understand Islam without good knowledge of the vehicle of its revelation through the Quran - i.e. Arabic, neither can you truly understand Arabic in depth without understanding Islamic ideology. Islam presents a complete worldview where every concept in existence is the product of a source, with the ultimate Source and Creator being Allah. There are words that have had their own literal meanings in general but which were given a new usage in the Quran. Hence, via the language of the Quran Allah redefined the meaning of some words or gave a new religious dimension to them, and this meaning was then used in Islamic terminology to denote certain things.
For example, the word Iman (the antithesis of "Kufr" - the noun from KFR) does not only mean faith/belief. It does mean faith, but its root (A-M-N) means reliability,
trustworthiness, safety and security. Therefore to have "Iman" (faith) also denotes the idea that one becomes safe and secure, where faith leads to safety and salvation. Because of underlying concepts, words can have many meanings.
Now, Kafara (whose root is K-F-R), is just such a word with many meanings, and some of its meanings come out of the Quranic usage of them. One of its original meanings is "to cover up", or the "the farmer who sows seeds" (i.e. covers seeds up). Now, taken from the famous Abbas Ali Nadwi dictionary, its many meanings are translated into English as follows:
- to hide, cover up, deny, renounce (can you see the logical continuation?)
- when used with the Arabic preposition "bi", it means to reject (opposite of belief). Kafara bi-Allah means to reject/disbelieve in Allah.
- to deny/disbelieve, for example the Quran uses it in this meaning in the following verse:
"And a group of the Children of Israel believed, and a group did kufr
- to be ungrateful, negligent (opposite of thankfulness). This meaning comes from the fact that God puts "k-f-r" as the opposite of "sh-k-r" in the Quran, which means gratefulness. For example in the following verse:
"Whosever gives thanks, he only gives thanks for (the good of) his own
soul, and whoever does kufr(is ungrateful) (is only ungrateful to his
own soul). " (27:40)
A conceptual understanding arises from here that recognition of one's Creator is a form of thankfulness to the Source who gave one everything that one has in life, from all the mechanisms of the universe that make life possible on earth, to one's own limbs and intellect and abilities and relationships and food and sustenance. This follows naturally from the idea that if someone were to give me a gift and I were to ignore their existence, it would be a kind of ungratefulness.
- to reject. For example, the Quran uses this word for the great Prophet Noah whom God saved on the Ark after he had spent over a good nine centuries of his life preaching to his people, who had consistently rejected him:
And We carried him on a (ship) made of planks and nails, Floating
under Our Eyes, a reward for him who had been kufir (rejected)!
- In one chapter, the Quran uses the word "Kuffar" (derived from KFR) twice within the same verse, once denoting those who disbelieve, and once denoting farmers who sow seeds.
There is also the word "Kaffaarah", which means a compensation, such as a penalty to be paid to "make up" for something, or "cover it up" if you will.
Going a bit deeper, there is the concept in Islam that God creates everyone equally, and every human being is born pure and in harmony with nature, in harmony with the Creator and His Creation, and hence is inherently "Muslim" (one who submits to God by nature). Converting to another religion is considered only a product of nurture. Therefore a person who is presented with the Truth of its Creator and rejects/disbelieves in it is actually "covering up/hiding" their true inner nature, or hiding the truth. (There is another discussion of the difference between one who rejects God outright (kaafir) and one who believes in God, but does so without believing in His true Sovereignty and by giving others a share in His Divinity, such as considering others to be God with Him. But that is another topic).
Now the thing is - if a single word can have so many meanings, you can image this might create problems in communication - because a person could mean many things at the same time. So what happens in regular communication is that usually one broad meaning is conventionally understood. When we're on the topic of religion, then in Islam, "Kaafir" most often simply means a "disbeliever" of Islam. This is the most common meaning used. Also, most Muslims today do not know any other meanings beyond that, unless they've formally studied Arabic and the Quran at a higher level.
As for the word Muslim, this comes from the root "SLM" as I answered in your previous question, which denotes meanings of peace and safety. It also refers to the concept of submission (which is the logical prerequisite to peace. After all, there is no peace where everyone rears their own head in self importance - there has to be some "giving up" and "surrendering").
Meanings of interrelated words derived from S-L-M, again from the valuable Abbas Nadwi dictionary that refers to the Quran, with minor edits:
- salima: to be in sound condition/ well/ without blemish.
- sallama means to hand over/transmit.
- sallama also means to salute/greet (usually to give the greeting of peace. "Assalamu alaikum")
- sallama also means to be solid/whole.
- sallam also means to save.
- aslama: to submit oneself to someone/something. Usually to accept Islam.
"Aslama" is simply a higher derivative verb form that conjugates from a triliteral root by prefixing the root with the letter "a". When this happens with any root, then the derived verb usually means that one is attaining the meaning of that root through one's own self (not by anyone else doing it for them). The verbal noun of "aslama" is "Islam".
The word "silm" means peace, reconciliation, self-resignation, submission, and with the definite article, this word is synonymous with the word "Islam", as it is used in this Quranic verse:
"Oh you who believe! Enter into al-Silm wholly." (2:208)
"Muslim" is the active participle of the verb "aslama", or one who chooses the state of "silm"/"Islam", so it literally means to willingly submit/surrender oneself/become at peace/become whole, and in Islamic terminology the meaning converges to "submit to Allah" thereby achieving peace/security. Long story short, "Muslim" is one who takes Islam (submission to God) as one's religion. This applies to a person no matter what time and place on earth they may be. If they were alive at the time of Prophet Noah, Abraham, Moses or Jesus (on them all be peace) and followed their original teachings of submission to God, or if they are alive after the time of Muhammad (peace be upon him) - i.e. in this era - and follow his teachings of submission to God, or if they were never exposed to any Prophet or guide but truly submitted their heart to Him anyway.