I once heard “Jan Kees” meant John the cheese eater, as the Dutch were fond of eating cheese ...
“Kaas” is Dutch for cheese. Kees (or sometimes Cees) sounds like “case” and is just a masculine name which is often hyphenated with Jan (John). The origin of “yankee” is somewhat uncertain, so maybe it was Jan Kaas, John the cheese eater.
I once encountered a professional singer (operatic type) who had the last name “Kaasschieter” which means “cheese shooter” . . . Oh boy, I don’t want to go any further there . . .