there was this guy once who was fucking his girlfriend upstairs. His mum called out "Johnny, do you want your tea", at which point he shouted "Yeah mum, im cumming!!, whilst releasing a sack load of cum up his bird.

If I'm not mistaken, it comes from the word "culminate"...then people shortened it to "culm"...lots of people just spell it cum. Culminate means to finish, or to reach the highest point, climax.

I dont know how that came to be... i am big on AIM and me and all my friends use "cum" as "come" all the time.

Many times during coitus, people will exclaim "I'm coming" or "he's coming" meaning, coming to orgasm; coming to climax. Over many years it was abreviated and shortened then made into a pronoun "cum" as another slang term for semen but also refers to a man or woman reaching orgasm or "coming" to orgasm.

Other slang words for semen:

Jism
Spunk
Cream
Honey
Wad

Spunktus bejunktus