Have you ever thought about the origins of the thumb signal? More commonly known as the “thumbs-up” or “thumbs-down”, these are probably the most commonly used hand gestures in the world. The thumbs-up gesture is generally linked with positivity, approval, and satisfaction, while the thumbs-down is associated with more negative things such as disapproval, dissatisfaction, and failure. It’s widely believed that the gestures originate from the gladiatorial combat of Ancient Rome, where two combatants fight each other in a bloody battle. The Roman Emperor would then decide the loser’s fate with either a thumbs-up or a thumbs-down. The thumbs-up means that the loser’s life will be spared, while the thumbs-down means that the victor will have to slay the opponent.

          The thumb signal is a physical form of what we call “orientational metaphors”. All languages rely on orientational metaphors, in which we assign emotional concepts onto spatial concepts. In English, for example, the direction “up” is considered good. This is why we say that happy things “pick us up” or “cheer us up”, and we love it when things are “looking up” because it might make us feel like we are “standing on top of the world”. On the other hand, the direction down is considered bad. We don’t like it if things “go downhill” or when we feel “under the weather”. The term “to fall off” is also used as an insult in modern internet lingo to signify the loss of one’s relevancy. Similarly, we also like it when we “stand on business”, but we sure don’t want our business to “go under”!

Sources:
http://penelope.uchicago.edu/~grout/encyclopaedia_romana/gladiators/polliceverso.html
https://glossary.sil.org/term/orientational-metaphor