During the gladiator combat, Ancient Roman crowds used to point their “thumbs up” if they wanted the defeated gladiator to be spared or “thumbs down” if they wanted him condemned to death! A 19th century artist, Jean-Léon Gérôme, painted a picture showing a gladiator looking up to the crowds, basically, asking them for their verdict on the gladiator.
References:
https://en.wikipedia.org/wiki/Thumb_signal
Gray’s Anatomy drawings