The "thumbs up" sign was originally used in Ancient Rome for crowds to signify to the winning gladiator that they wanted him to kill his opponent.
In Ancient Rome, crowds used the thumbs-up sign to the encourage the winning gladiator to actually kill his opponent.
In addition, the thumbs-down meant to spare the defeated. This signal was used for a similar purpose in World War II, when it was used to signify acknowledgment of a good kill between two pilots.
