"club-shaped instrument used for pounding and breaking materials in a mortar," mid-14c. pestel, (as a surname late 13c.), from Old French pestel and directly from Latin pistillum (Medieval Latin pestellum) "pounder, pestle," related to pinsere "to pound," from PIE *pis-to-, suffixed form of root *peis- "to crush" (source also of Sanskrit pinasti "pounds, crushes," pistah "anything ground, meal," Greek ptissein "to winnow," Old Church Slavonic pišo, pichati "to push, thrust, strike," pišenica "wheat," Russian pseno "millet").
Also in old use "the leg of certain animals used for food" (14c.), hence pestle of a lark "a trifle, an unimportant matter" (1590s).