verb ("to ejaculate") and noun ("semen"), by 1973, apparently a variant of come in the sexual sense that originated in pornographic writing, perhaps first in the noun. 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,
vntill I saw her vnder.
then off he came & blusht for shame
soe soone that he had endit;
yet still shee lyes, & to him cryes,
"Once More, & none can mend it."
As a noun meaning "semen or other product of orgasm" come is attested by the 1920s.
The sexual cum seems to have no connection with Latin cum, the preposition meaning "with, together with, in connection with" (an archaic form of com; see com-) which English uses on occasion in names of combined parishes or benefices (such as Chorlton-cum-Hardy), in popular Latin phrases (such as cum laude), or as a combining word to indicate a dual nature or function (such as slumber party-cum-bloodbath).