To get (a passing vehicle) to stop and give one a ride, signalling to the driver by holding out one's thumb (as a hitchhiker)
I was stranded in the middle of nowhere, but luckily I managed to thumb down a passing car.
To disapprove or reject
A gesture signifying disapproval; a thumb pointing down out of a fist.
He shook his head and gave a thumbs down.
A rejection or refusal.
We requested permission but got a thumbs-down.