Definition from Wiktionary, the free dictionary
Alternative forms 
With reference to the signal of approval or disapproval used by spectators at a Roman amphitheatre, but sense has been reversed.
- A gesture signifying approval or okay; a thumb pointing up out of a fist.
- He smiled and gave a big thumbs up.
- (idiomatic) An approval or okay.
- We got the thumbs-up to go ahead with the project.