Tarkil was a Quenya word, borrowed by speakers of the Common Tongue to describe the Men of the West, those noble Men of Númenórean descent. The word came ultimately from a root form thought to be tára-kil, meaning literally high follower (the Elves referred to Men as followers because they were the Afterborn Children of Ilúvatar).
The Orcs twisted the word to their own ends, and as Tark they used it as an insulting term for the Gondorians. 'Tarkil' also probably lies behind the meaning of the name of Tarcil, the sixth King of Arnor, one of Aragorn's ancestors and an Heir of Isildur in his own right. In The Lord of the Rings, the word is seen only in a note in Appendix F, though at one time it was destined for more prominence: in the early drafts of the work, Aragorn himself is known as 'the Tarkil', but this is changed to 'the Dúnadan' in the published text.