Expressions of approval and disapproval respectively: “The two critics disagreed about the movie; one gave it thumbs up, the other thumbs down.” In the gladiatorial contests of ancient Rome, a thumbs-up gesture from the crowd meant that the loser would live; thumbs down meant death.
An expression of approval or hopefulness, as in The town said thumbs up on building the elderly housing project. The antonym
thumbs down indicates disapproval or rejection, as in Mother gave us thumbs down on serving beer at our party. Alluding to crowd signals used in Roman amphitheaters, these idioms were first recorded in English about 1600. In ancient times the meaning of the gestures was opposite that of today. Thumbs down indicated approval; thumbs up, rejection. Exactly when the reversal occurred is not known, but the present conventions were established by the early 1900s.