num-nut

someone who says something stupid.

This usually refers to someone who says something stupid in a particular point of time. This does not necessarily mean they are dumb, they just said something dumb at that time which makes them a num-nut.

Person 1: Hey man, what is the date of fourth of July?
Person 2: You are joking right?
Person 1: ohhhh hahaha nevermind
Person 2: gosh, your are such a num-nut