Green’s Dictionary of Slang

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

1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1864, 1867, 1870		Hotten Sl. Dict. 141: GASSY, or gaseous, liable to ‘flare up’ at any offence.
1873		Sl. Dict.
1890–1904		Farmer & Henley Sl. and Its Analogues.

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

1842		Whip & Satirist of NY & Brooklyn (NY) 30 Apr. n.p.: It talks very loud and uses some threats [...] the weakly, gassy Roarer [a newspaper].
1848		Ladies’ Repository (N.Y.) Oct. VIII:37 316/2: Gassy, given to boasting.
1850		Manchester Spy (NH) 7 Sept. n.p.: The communication signed shooting stick is too gassy for our columns.
1864		‘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’.
1878		‘’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.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1900		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.
1903		‘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.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 77/1: Gassy, a. Talkative.