Green’s Dictionary of Slang

to perform an action, esp. in context of violence.

1887		G. Devol Forty Years a Gambler 10: He let go and hit me a slap in the face that made my ears ring; so into him I pitched.
1924		D. Hammett ‘Death on Pine Street’ in Nightmare Town (2001) 213: I reached up and let go. He settled in a rather limp pile.
1935		D. Lamson We Who Are About to Die 193: So the kid lets go with the poker, an’ Mike goes out like a light.
1970		J. Wambaugh New Centurions 124: ‘I [...] took out the baton and broke two of his ribs and they had to put thirteen stitches in his head. [...] I never done that before or since. I mean I never let go before’.