This self-referential American term is of uncertain origin. The most likely source of the word is the Dutch nickname Janke, a diminutive of Jan (John). Although it must be said that this is not the only possible explanation.
Use as a nickname for a person is attested to from the 1680s. The Calendar of State Papers, Colonial Series, an archive of British government documents, has this from 1683:
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They sailed from Bonaco..; chief commanders, Vanhorn, Laurens, and Yankey Duch.
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