This answer is more likely to be an assumption, as I'm neither a specialist in the Order of tanzil nor could I find a specific Question on the topic so far. But I could find some links giving a certain order of tanzil (revelation) with complete Suwar/Surahs. AFAIK this order may differ when it comes to Verses, so maybe a Verse of Surah A has been revealed before another Verse from Surah B (which might be revealed or completed later then A) or A itself. So I think it's hard to tell whether the following is correct:
Assuming the Order of the Suwar/Surahs of the Qur'an on the pages I linked is correct, and also assuming that the Suwar have been revealed completely before each other I would say the most first appearance of this word would be in Surat al-Qalam (68:51), where the verb (disbelieve) appears and which most Scholars agree that it's the 2nd Surah which has been revealed. Then comes be Surat al-Muddathir (74:10) where the noun (disbelievers) appears! Surat al-Muddathir is considered the 4th in the revelation order.
Here are two links 1 & 2 about the order of revelation of the Suwar (only in Arabic so far)
And Allah knows best!