I saw a documentary many years ago that had an interesting and very plausible explanation of the thumbs down gesture. What prompted this theory was the discovery of several gladiator skeletons that, naturally enough, showed all sorts of battle injury; however, there was one injury that was both common and difficult to explain: many squeletons had damage due to a sharp instrument to the interior side of the backbone.
The conclusion they offered was that these cuts were made by a bladed weapon coming from the above the rib cage, down into the chest cavity. In other words, a death blow from a standing gladiator with a gladius upon another kneeling gladiator. Such a stabbing motion would cut several large blood vessels, might even reach the heart, and sometimes nick the backbone.
The people who interpreted this discovery concluded that the “thumb down” gesture, was probably more of a thumb-to-the-throat-pointing-down-into-the-chest gesture.
I apologize for not remembering the source of this information, but I’m hopeful that someone can recall the documentary and some of the people who offered this analysis.
This is from Page 76 of “The Book of General Ignorance”
"… in 1997 a Roman medallion of the second or third century A.D. was discovered in southern France. It shows two gladiators at the end of a battle and a referee pressing his thumb against a closed fist. The inscription reads, “Those standing should be released.”
The article is of the opinion that thumb down is spare him and thumb up is kill him.
The version I heard was that thumb down meant “Drop your sword”, ie, spare him, and that thumb toward the body or neck meant “Stab him”.
I have also seen diagrams of the deathblow that Momotaro described. The victor would stab through the trapezius muscle, between the clavicle and the scapula, going straight to the aora or the heart.
According to Desmond Morris, the sign to kill was a stabbing motion made with a downward turned thumb. The sign to let him live was a thumb enclosed by the other four fingers, meaning “sheathe the sword.”