verb and noun, by 1973, apparently a variant of the sexual sense of come that originated in pornographic writing, perhaps first in the noun sense. This "experience sexual orgasm" slang meaning of come (perhaps originally come off) is attested from 1650, in "Walking In A Meadowe Greene," in a folio of "loose songs" collected by Bishop Percy.

They lay soe close together, they made me much to wonder;
I knew not which was wether, until I saw her under.
Then off he came, and blusht for shame soe soon that he had endit;
Yet still she lies, and to him cryes, "one more and none can mend it."

As a noun meaning "semen or other product of orgasm" it is on record from the 1920s. The sexual cum seems to have no connection with Latin cum, the preposition meaning "with, together with," which is occasionally used in English in local names of combined parishes or benifices (e.g. Chorlton-cum-Hardy), in popular Latin phrases (e.g. cum laude), or as a combining word to indicate a dual nature or function (e.g. slumber party-cum-bloodbath).