Bite your tongue

A straightforwardly physical image turned idiom: literally clamping your teeth down on your own tongue to physically stop yourself from speaking. It's recorded from the 16th century, and Shakespeare clearly liked it, using versions of the phrase in three separate plays, including Henry VI, Part 2 ("So York must sit and fret and bite his tongue") and Titus Andronicus. The meaning has stayed essentially unchanged for over four hundred years: holding back words, usually out of tact, restraint, or self-preservation, that you'd otherwise blurt out.