The world’s friendliest hand signal—the one we flash for “good,” “yes,” and “we’re cool”—probably started as a Roman death sentence. A 19th-century painting flipped the meaning, Hollywood locked it in, and wartime pilots finished the job. Now that same upward thumb ends text threads and clears roller-coaster trains like nothing ever happened.

The thumbs-up gesture that means “good,” “yes,” and “we’re cool” may have started as a Roman death sentence. One famous 19th-century painting flipped the meaning, Hollywood locked it in, and wartime pilots finished the job.

You’re standing in a noisy bar in 1944, flagging a ride in the seventies, or doom-scrolling at 2 a.m. when a yellow hand with the thumb stuck straight up appears. Instantly you know: good, yes, all clear. Except there’s a solid chance that exact same motion once meant the op ...