Surely, the questioner would find it easier to look the word up in a decent dictionary (one that gives etymologies) rather than go through the rigmarole of posting a question on this website? Chambers Dictionary (they miss out the apostrophe in the title) says it is from the Dutch everyday word for John or Johnny, and, strictly, only refers to a citizen of the New England states, or the northern United States.
It is possible that it came from a Native American word, but it is far more likely - as the earlier answer suggests - to have come from the Dutch word 'Janke', meaning 'Johnnie'...('J' is pronounced like 'Y' in Dutch, so it sounds more like 'Yanke' to British ears)...or even from 'Jan Kees', meaning 'John Cheese'. Apparently that was used as a term of abuse by Dutch settlers when referring to English ones in the early days of the USA.