Originally, 'Yankee' referred to Union soldiers in the American Civil War. My great-grandfather was a Yankee who fought in the Civil War.
Pro Tip 2/3
Regional Nuance
'Yankee' can imply different political and cultural attitudes associated with the North. As a New England yankee, she preferred maple syrup to molasses.
Pro Tip 3/3
Not Always Flattering
In the South, 'Yankee' might be used with a slightly negative tone. Some southerners still tease him for being a yankee.