Green’s Dictionary of Slang

(US) a large pocket knife with folding blades.

1846		N.Y. Dly Tribune 10 Mar. 2/4: To-day Mr Barlow — not ‘Billy’ of knife memory, but equally celebrated in his way — made a speech.
1875		N. Ohio Jrnl (Painesville, OH) 2 Oct. 4/4: ‘Yonder comes Billy Barlow,’ said Snags. ‘I’ll bet he’ll want trade his old barlow-knife for some berries. He’s been tryin’ for a year to get somebody to trade something [...] for that old broken-bladed, rickety knife’.
1874		‘Old Calabar’ Won in a Canter I 150: Charles Barlow, the celebrated fishing-tackle maker, and first cousin of the equally well-known ‘Billy Barlow’.
1880		Sacramento Dly-Record Union (CA) 22 Nov. 4/4: A great many of us were happy to be remembered on Christmas Eve with a penny whistle or a ‘Billy Barlow’ knife.
1902		Free Lance (Frericksburg, VA) 28 Oct. 5/2: I used to know an old jingle that ran, ‘A Barlow knife and a Barlow blade, / The best old knife that ever was made’ .
1908		Wash. Herald (DC) 1 Jan. 10/7: It was the same old Billy Barlow knife his mother had given him thirty years before.
1914		Clay City Times (KY) 30 Apr. 1/5: A Billy barlow, with half the blade broke off.
1919		Abbeyville Press & Banner (SC) 12 Dec. 6/1: The billy barlow given him by a co-conspirator for self-defence.