This is one that I had wondered about for a while now. Having been from Illinois and moving to Georgia, I hear it a good deal, even after living here for 11 years.

This is reported to be Dutch in origin. Fishing boats from Holland often fished off of the coast of what is now New England. Since many of the early settlers in New England and New York were Dutch, and so many of them had the common sur-name of 'Jan', the plural form of which was 'Janke'. Due to the peculiarities of Dutch pronunciation, 'J' is commonly sounded as 'Y' to English speakers. It has since become broadly applied to any New Englander, and eventually, to any North American.