Green’s Dictionary of Slang

1. (US) an Irishman.

1893		F. Harris ‘Gulmore, the Boss’ Elder Conklin & Other Stories (1895) 174: There’s quite a number of Mugwumps, an’ if the Professor goes about workin’ them all up – what with the flannel-mouths and the rest – it might be a close finish.
1904		C. Connors Bowery Life [ebook] Say, don’t you know what a chaw is? He’s a mug wid a sponge in his mout’ you know; a flannel-mout’ bloke.
1913		J. London Valley of the Moon (1914) 25: Hey, old flannel-mouth! Watch out! You’ll get yours in a second.
1930		‘Dean Stiff’ Milk and Honey Route 38: An Irishman is a ‘flannel mouth,’ ‘a chaw,’ or a ‘mick’.
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 245: Hello, Flannel Mouth! [Ibid.] 268: Keefe, you drunken flannel-mouth .
1949		Monteleone Criminal Sl. (rev. edn).
1951		S. Lewis World So Wide 245: But then I saw you all hated me and despised me for bawling him out – shanty Irish, flannel-mouth, nuisance!
1964		Amer. Dialect Society Publications XLII: Chaw-mouth . . . refers to the Irishman’s talkativeness and parallels the more common flannel-mouth [R].
1966–67		in DARE.
1997		P.F. Morris Anaconda Montana 110: The average flannel mouth loves to talk about freedom.

2. (US) a loudmouth, a braggart, one who talks too much and with too little sense; also as term of address.

1911		Van Loan Big League 165: You’ve let these knockers get you going [...] Show these flannel mouths what a regular outfielder looks like.
1928		Hecht & MacArthur Front Page Act III: That tub of guts! Lousy whore-headed flannel mouth!
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 246: ‘Hell, Flannel Mouth! How’s the brother?’ asked Studs.
1938		D. Runyon ‘That Ever-Loving Wife of Hymie’s’ in Runyon on Broadway (1954) 606: I hear what this big flannel-mouth says about the price.
1947		N. Algren Neon Wilderness (1986) 68: The bartender was a flannelmouth. Everyone in the ward was a flannelmouth.
1951		P. Whelton Angels are Painted Fair 74: Which one do you mean, flannel-mouth?
1968		K. Brasselle Cannibals 259: You can’t give the script to anyone who has a few drinks and a big flannel mouth.
2004		B. Devine Memoirs 26: Dick was a great guy, but he was a flannel mouth. He flapped his mouth. He talked too much.

3. (Can./US) a well-spoken person.

1984

Partridge DSUE (8th edn) 400/2: Can. C.20.

4. (US) one who garbles their words.

1938

H.M. Anderson Strip Tease 43: If the straight man has false teeth and globbers his lines, he’s a ‘flannel mouth’.

5. (US) a Pole.

1942

N. Algren Never Come Morning (1988) 87: ‘Flannel-mout’!’ Silence. He grew bolder in his contempt. ‘Swinia! German!’.