During anal sex, a cum bubble is produced when a male ejaculates into the rectum of his partner. Since it feels like flatulence, which is essentially a bubble of gas in the rectum, it was thus named.
The gargling of jizz immediately following a blowjob over the blowee's dick in a bubbly, spitty manner accompanied by a forceful neigh. (Picture a blower with a mouthful of dick and freshly erupted jizz imitating precisely the sound of an excited horse.)
Dude, my girl cum humphreyed me so loud last night that the neighbors phoned local authorities. I had jizz spattered all over my Tommy Hilfigers.