A Few Slices Short Of A Loaf Of Bread

Somebody who's not quite mentally competent
Similarly you may use the phrase, a few fries short of a happy meal

Weird Bus Dude: Hey wanna be my friend????? (waves spastically)
B: Can't Say I do
S: Well that guy is a few slices short of a loaf of bread!

By Aidan

Define A Few Slices Short Of A Loaf Of Bread Meaning