Green’s Dictionary of Slang

flannel-mouthed adj.

also flannel-tongued
[predates flannel n.2 ; note SAmE flannelmouth, a very large catfish]
(orig. US)

1. having a large mouth.

[US]‘Bill Nye’ Bill Nye and Boomerang 155: The [...] lantern-jawed, sway-backed, mangy, flannel-mouthed poet of the educated and refined East.

2. (also flannel-mouth) loud-mouthed.

[US]Oskaloosa Herald 17 Mar. in Miller & Snell Why the West was Wild 311: The mayor is a flannel mouthed Irishman and keeps a saloon and gambling house.
[US]Intermountain Catholic (Salt Lake City) 5 Oct. 4/3: The same stupid vulgarities concerning [...] flannel-mouthed Irish and chicken-stealing niggers.
[US]F. Hutchison Philosophy of Johnny the Gent 79: ‘ He's a big flannel mouth rummy that's been readin’ some o' them Hawkshaw the Sleuth tales’.
[US]Ersine Und. and Prison Sl.
[US]N. Algren ‘Lightless Room’ in Entrapment (2009) 44: He yelled over to me I was just one more flannel-mouth Polack.
[US]E. O’Neill Long Day’s Journey into Night II ii: That flannel-mouth, gold-brick merchant.
[US]N. Algren Never Come Morning (1988) 44: He’d get them all in trouble some day with that flannel-tongued trap of his.

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

[US] (ref. to mid-19C) N. Kimball Amer. Madam (1981) 23: The family was always hard and fast church goers, always with its ass out of its jeans, flannel-mouthed and proper.

4. talking thickly or with a brogue [the idea of talking with a SE flannel in one’s mouth].

[US]E. Hemingway letter 1/12 Nov. in Baker Sel. Letters (1981) 344: This damned typer skips like a stammering flannel mouth nigger.