I'm curious about the origin of and explanation for the place-index-finger-to-closed-lips gesture. All I've found so far is this unattributed assertion that it dates at least to the era of Classical Rome.
For instance, consider two gestures of similar meaning:
With one's thumb against one's curled index finger, twisting the hand in front of one's closed lips. Plainly, this gesture is iconic, suggesting the act of turning a key in a lock to convey the meaning, "My lips are sealed."
Again with thumb and index finger pressed together, move them along one's closed lips. This gesture too is iconic. Here it's the act of closing a zipper (in UK, a zip), and conveys the similar, "I'll keep my lips zipped."
But is the shush gesture iconic? If so, then of what? Also, how widely distributed across cultures is its use? We're told (vide supra) that the Romans used it...