Green’s Dictionary of Slang

marbles n.3

[resemblance, but note marble halls n.]

the testicles.

[UK]‘Night in a Country House’ in Cabinet of Venus 68: My left hand slowly feeling those marbles that hung down.
[US]H.N. Cary Sl. of Venery.
[US]L. Kramer Faggots 147: Another naturally played with his young marbles.
[US]H. Rawson Dict. of Invective (1991) 343: lost (or missing) some (or one’s) marbles (possibly from the old belief [...] that masturbation causes idiocy, since marbles are also testicles and to lose [one’s] marbles also means to experience orgasm.
[UK]M. Manning Get Your Cock Out 22: I’m trying to get me facking marbles drained here!
[UK]Roger’s Profanisaurus in Viz Apr. 49: trouser tits n. Man marbles.

In phrases

crack one’s marbles (v.) (US)

of a man, to achieve orgasm or to induce it in a woman.

[US](con. 1930s) R. McKenna Sons of Martha 145: What you buy in Frisco is just the orgasm [...] If they can crack a pair of marbles every two minutes, why, that’s just the old Henry Ford spirit.