"Betise" is general folly, imbecility, stupidity, foolishness; an error, mistake, or fault; attributable to bad judgment, ignorance or inattention. The English word comes from the French bêtise (stupidity, nonsense), from bête (meaning either "foolish" or "beast"), from Old French beste (beast), originally from Latin bestia (beast, obviously).
Oddly enough with that meaning, Betise is occasionally used as a woman's first name. Betise also seems to be a couple of French restaurants in the Chicago, Illinois area and a resort in the Dordogne, France. "Betises de Cambrai" are a type of candy from the north of France and named after a town there (I'm told they're like humbugs).
In an unrelated and non-European usage, "Betise" is a dialect of the Besisi language spoken by a small number of people on the peninsula of Malaysia.