piston (n.)

in mechanics, "a movable piece so fitted as to fill the section of a tube and capable of being driven alternately in two directions by pressure on either side," 1704, from French piston "a piston," in Middle French "large pestle," from Old Italian pistone "a piston," variant of pestone "a pestle," from pestare "to pound," from Late Latin pistare, frequentative of Latin pinsere (past participle pistus) "to pound" (see pestle). As a verb, "to move like a piston," from 1930.