mouth·piece / ˈmou[unvoicedth]ˌpēs/ • n. 1. a thing designed to be put in or against the mouth: the snorkel's mouthpiece. ∎  a part of a musical instrument placed between or against the lips. ∎  the part of a telephone for speaking into. ∎  the part of a tobacco pipe placed between the lips. ∎  a mouthguard.2. chiefly derog. a person or organization that speaks on behalf of another person or organization: they become nothing more than a mouthpiece for the company. ∎  inf. a lawyer.

mouthpiece (Fr. bec (woodwind); embouchure (brass)). Part of a woodwind or brass instr. which is inserted in the player's mouth or to which he applies his lips in order to produce a sound.