Green’s Dictionary of Slang

1. a shotgun.

1892		in Punch 8 Oct. 159: It sometimes creates a pretty effect to call your gun ‘My old fire-iron,’ or ‘my bundook,’ or ‘this old gas-pipe of mine.’.
1899		G.H. Russell Under the Sjambok 212: I would become possessed of a weapon on the first opportunity; even an old ‘gas-pipe’.

2. in pl., tight trousers.

1909		J. Ware Passing Eng. of the Victorian Era.
1922		Aberdeen Press & Jrnl 1 July 4/6: [He] would probably be known among the deckhands as a bit of haw-haw. and he would almost certainly be wearing gaspipes (close-fitting trousers).

3. (US) a slide-trombone.

1932		J.L. Kuethe ‘Johns Hopkins Jargon’ in AS VII:5 332: gas pipe—slide trombone.
1944		‘Jiver’s Bible’ in D. Burley Orig. Hbk of Harlem Jive.