Q: Here’s a quick one for you: In the book I’m writing, I have a character say, “I gave him the thumbs up.” Why do we use the plural “thumbs” in this expression when we use only one thumb to make the gesture?
A: Why the plural? Because when the expression originated, it referred to the many people (a coliseum full of them, in fact) who were voting with their thumbs. But back then, “thumbs up” was bad news.
Under its entry for “thumb,” the Oxford English Dictionary notes that “thumbs down” and “thumbs up” were originally “expressions referring to the use of the thumb by the spectators in the ancient amphitheatre, to indicate approbation or the opposite.”
In the time of the Romans, “thumbs down” signaled spare him, while “thumbs up” was a death warrant.
In modern usage, the significance of the signals has been reversed, according to the OED, so “thumbs down” now means “disapproval or rejection,” while “thumbs up” is “a sign of approval, acceptance, encouragement, etc.”
Rudyard Kipling, for example, used the modern sense in Puck of Pook Hill (1906), “We’re finished men – thumbs down against both of us.”
So did Arthur Guy Empey in a 1917 glossary of terms used in the trenches: “Thumbs up, Tommy’s expression which means ‘everything is fine with me.’ “