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1926, in jazz slang, in reference to the effect on brass instruments made by manipulating the mute; of imitative origin. Later also in reference to an electric guitar effect. As an imitation of the sound of a baby crying, it is recorded from 1938. Wah-wah pedal is recorded from 1969. Cf. Chinook jargon wawa "talk, speak, call, ask, sermon, language;" Cree (Algonquian) wehwew "goose," Lenape (Algonquian) wava "snow goose," all probably of imitative origin.
To talk: to be seen waggin' your chin with a sleuth (1920+)