Green’s Dictionary of Slang

1. to allow anything, real or fig., to go at full speed, to remove any impediment to progress.

[1862		Rocky Mountain News (Denver) 31 May n.p.: And always made it his exclusive ‘biz’ / To mingle in a crowd and ‘let ’er whiz;’ / To shoot at random was a heap of fun].
1864		J.F. Brobst letter in Brobst Well Mary, Civil War Letters 108: With a fast trotter [...] plenty of snow and a gay handsome girl by one’s side. Two forty on the plank, let her went!
1889		Literary News 9-10 177: The parrot who astonishes the rustic fowler by shouting ‘Let her go, Gallagher!’ .
1890		E.O. Kirk Maidens Choosing 449: Let her go, Gallagher, but let her go slow.
1904		Sydney Sportsman (Surry Hills, NSW) 24 Aug. 3/3: Melinda Jane must be a physical culture crenk, with muscles like Harry Cansdell. Let her go, Gallagher.
1905		G.W. Peck Peck’s Bad Boy Abroad 81: Act as though you were going to jump overboard, and then stop sudden and let-’er-go-gallagher, right before folks.
1909		Star (Sydney) 8 July 13/3: ‘I let her go Gallagher, big uppercut, and hit Schmidt in de belly’.
1912		Bemidji Dly Pioneer (MN) 4 Oct. 6/3: The expression ‘Let her go, Gallagher’ is in use in nearly every city of the United States.
1915		H.E. Rollins ‘A West Texas Word List’ in DN IV:iii 226: go Gallagher, adv. phr. In expressions such as – ‘It’s going to rain.’ ‘Well, let her go Gallagher’.
1926		Wood & Goddard Dict. Amer. Sl.
1945		J. Henderson Gunner Inglorious (1974) 76: ‘Let her go, Ray,’ we said. ‘Fire ahead, boy. Get it off your chest; it will do you good.’.
1951		S. Longstreet Pedlocks (1971) 393: ‘Let ’er go,’ said Joey.
1965		W. Dick Bunch of Ratbags 232: Right-oh [...] let her go.

2. (US) to vomit.

1940

(con. 1918) S.J. Simonsen Soldier Bill 52: Bill [...] went over to the garbage can and ‘let her go’.