Green’s Dictionary of Slang

gassy adj.1

[fig. uses of SE + gas v.1 (1)]

1. (also gaseous) irascible, likely to ‘flare up’ without warning.

[UK]Hotten Dict. of Modern Sl. etc. (2nd edn).
[UK]Hotten Sl. Dict. 141: GASSY, or gaseous, liable to ‘flare up’ at any offence.
[UK]Sl. Dict.
[UK]Farmer & Henley Sl. and Its Analogues.

2. (also gas) talkative, verbose, boastful; also as a nickname.

[US]Whip & Satirist of NY & Brooklyn (NY) 30 Apr. n.p.: It talks very loud and uses some threats [...] the weakly, gassy Roarer [a newspaper].
[US]Ladies’ Repository (N.Y.) Oct. VIII:37 316/2: Gassy, given to boasting.
[US]Manchester Spy (NH) 7 Sept. n.p.: The communication signed shooting stick is too gassy for our columns.
[US] ‘Cong. Globe’ in DN IV:i 45: [The man] has the reputation of being a most notorious liar, so much so that he went by the name of ‘gassy Brown’.
[UK] ‘’Arry to the Front!’ in Punch 9 Mar. 100/2: But Charlie, old chip, there’s a Party, a nasty, mean, snivelling gang, / Led by that gassy old Gladstone.
[UK]Farmer & Henley Sl. and Its Analogues.
[US]A.H. Lewis ‘Gassy Thompson’ in Sandburrs 29: ‘Gassy’ Thompson he was called by those about him in testimony to his powers as a conversationalist.
[US]‘Hugh McHugh’ Out for the Coin 17: Say, you’re handing me the same line of gas gab that Uncle Peter threw at me this morning.
[US]Monteleone Criminal Sl. (rev. edn).
[US]Goldin et al. DAUL 77/1: Gassy, a. Talkative.