"thumbs down" の定義と意味

What is the origin of the idiom "thumbs down" and when to use it?

The idiom "thumbs down" originated from ancient Roman culture. When gladiators fought to the death in arenas, the crowd would use gestures to indicate if a defeated gladiator should live or die. Thrusting a thumb downward meant death, so over time this expression came to symbolize disapproval or a negative judgment.