dummkopf

There're many phrases for someone who either isn't smart or right in the head. For example:

"lost his marbles",
"the engine's running but no one's behind the wheel",
"a few cards short of a full deck",
"bats in the belfry",

boxes of rocks, sharpest knives, brightest bulbs... yadda yadda. Some of these phrases date back before medieval England. However, we still have Cletuses and Jane Eyres in the modern age, so kind soul that I am, I thought I'd come up with a 21st century phrase.

"Low on Minutes."

You heard it here first.