@10 actually, it is in the dictionary: http://www.dictionary.com/browse/cum?s=t
uses "real" dictionary sources -- earliest published occurrence is 1650, is that respectable enough for Dan to start spelling it like EVERYONE does? :)
"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."