A thumbs up in most contexts simply means ok, good job, yes, or good. On the road, a thumbs up is a plea for a ride. In ancient Rome, it was Caesar’s decision to kill in the gladiatorial games. The thumbs up is encoded by the sender and the simple message is decoded as non-verbal communication by the recipient. The same sign (thumbs up) in different contexts yields different experiential interpretations.
I wrote about this recently, too — how we’ve become controlled by symbols like the thumbs up as predicted by Jean Baudrillard.