smart aleck, a

An impudent or obnoxiously self-assertive individual, a wise guy, as in New teachers often have a hard time coping with the smart alecks in their classes. This expression, dating from the mid-1800s, probably alluded to a person of this description who was named Alec or Alexander, but his identity has been lost.

A cocky individual who thinks he or she knows everything and is not shy about saying so. The origin of this term, an American colloquialism from the 1860s, has been lost; no one knows who that first Aleck (or Alexander) was. The adjective smart in the sense of “impudent” is much older (fifteenth century) but has died out except in this expression. Budd Schulberg used it in What Makes Sammy Run? (1941): “He’s a smart aleck. I can see already he thinks he knows more than I do.”