Green’s Dictionary of Slang

1. to reach orgasm.

1937–84		Partridge DSUE (1984) 677: C.19–20.
1970		San Diego Sailor 79: I saw him stiffen and let go with his.
2004		R.G. Barrett Rosa Marie’s Baby (2013) [ebook] Les let go with a howl that rattled the windows.

2. to spend money.

1902

Ade Forty Modern Fables 129: He Let Go rather freely, and the first thing he knew his Letter of Credit began to look lop-sided.

3. (US) to be quiet; usu. as imper. ‘forget it!’.

1919

Van Loan ‘Chivalry in Carbon County’ in Score by Innings (2004) 320: ‘I didn’t know you could ride at all!’ I presisted cruelly. ‘Aw, let go!’ pleaded Bud.

4. to urinate.

1964

J. Thompson Pop. 1280 in Four Novels (1983) 371: I was about to let go in my britches.

5. to shoot a weapon.

1973		E.F. Droge Patolman 144: ‘I heard you were shot at tonight.’ ‘Yeah, Ed [...]’ [...] ‘Did you let any go at him?’.
1994		M. McAlary Good Cop Bad Cop 43: Another [policeman] began to yell, ‘Shoot him. Shoot him.’ Joe, standing to the right, was also ready to let one go.