Today's OED antedating: "loud mouth"

Loud mouth, slang, pejorative, blusterer ("loud," a., sec 6, first use cited 1934): Book titled X Marks the Spot: Chicago Gang Wars in Pictures, by anonymous, 1930, p 40, col 1: "Capone had left the management of his empire largely in the hands of ... Harry 'Greasy Thumb' Gusick, convicted pander who had charge of a choice killing squad. Harry was ably assisted by Hymie 'Loud Mouth' Levine."