1683, a name applied disparagingly by Dutch settlers in New Amsterdam (New York) to English colonists in neighboring Connecticut. It may be from Dutch Janke, literally "Little John," diminutive of common personal name Jan; or it may be from Jan Kes familiar form of "John Cornelius," or perhaps an alteration of Jan Kees, dialectal variant of Jan Kaas, literally "John Cheese," the generic nickname the Flemings used for Dutchmen.
[I]t is to be noted that it is common to name a droll fellow, regarded as typical of his country, after some favorite article of food, as E[nglish] Jack-pudding, G[erman] Hanswurst ("Jack Sausage"), F[rench] Jean Farine ("Jack Flour"). [Century Dictionary, 1902, entry for "macaroni"]
Originally it seems to have been applied insultingly to the Dutch, especially freebooters, before they turned around and slapped it on the English. A less-likely theory (attested by 1832) is that it represents some southern New England Algonquian language mangling of English. In English a term of contempt (1750s) before its use as a general term for "native of New England" (1765); during the American Revolution it became a disparaging British word for all American natives or inhabitants. Contrasted with southerner by 1828. Shortened form Yank in reference to "an American" first recorded 1778. Latin-American form Yanqui attested in English by 1914 (in Mexican Spanish by 1835).
The rule observed in this country is, that the man who receives that name [Yankee] must come from some part north of him who gives it. To compensate us for giving each other nicknames, John Bull "lumps us all together," and calls us all Yankees. ["Who is a Yankee?" Massachusetts Spy, June 6, 1827]