|Lose your marbles|
Anyone know where this comes from.|
That and a lot of other euphemisms for lack of good reasoning ability have come about in recent years, mostly, I'd guess, on an ad-hoc basis by people consciously trying to put a humorous spin on the issue. None of these really relate to reality. "He's not got a full seabag," "He's a bit short of a full quart," other examples of something being just "short" of where it should be to be normal. Better than saying that he's a borderline mental defective, and a lot more humorous too.|
You can make up your own as you go along.