Though other comments appear to have found the true, Lisa-derived history, I remember a great explanation from "Macworld Mac Secrets" (quite the noble tome; it

Though other comments appear to have found the true, Lisa-derived history, I remember a great explanation from "Macworld Mac Secrets" (quite the noble tome; it certainly fed me full of tidbits about lesser-known features) Their explanation was that it looked like a finger pressing on a key in the way people do when pressing modifier keys, searching with their other hand for the correct normal key to press.