Elaine from Boulder, Colorado, asks why to clock someone means to hit them. The slang appears to come from clock as a noun for the face, attested by the late 1800s, with the verb for punching someone in the face showing up by the 1920s. A related...
The slang phrase XYZ, meaning “examine your zipper,” has been used since at least the 1960s as a subtle tipoff to let someone know his zipper is down. A variant, XYZ PDQ, means “examine your zipper pretty darn quick.” Other surreptitious suggestions...