garmouth, v. — Green’s Dictionary of Slang

garmouth v.

[SE gar, a fish that is generally considered not worth eating other than in the direst extremity]

(US) to boast, to brag, to make empty threats.

1984

K. Weaver Texas Crude 55: To gar-mouth. To spout empty threats and imprecations. ‘I’ll knock your ass up so high you’ll have to climb a stepladder to shit!’ is a class gar-mouth line.