Green’s Dictionary of Slang

a talkative person; thus a derog. term for an Irishman [alleged talkativeness of the Irish].

1882		Omaha Dly Bee (NE) 3 Apr. 2/3: There is just about as much sense in appointing ‘Chaw-Moth’ Cornell United States Commissioner as there would be in maling ‘Whiskey jack’ police judge of Omaha.
1893		Arizona Wkly Citizen 20 May 1/6: I’m ging to leave a mighty good man behind [...] It’s not a chawmouth, nor an Eyetalian.
1928		Rodman Yarns of a Kentucky Admiral 41: Knowing him to be a sea-lawyer, a ‘chaw-mouth’ and trouble breeder.
1964		American Dialect Society Publications XLII: Chaw-mouth . . . refers to the Irishman’s talkativeness and parallels the more common flannel-mouth.
1967		in DARE.