A reference to a man's jizz-load or load of cum after it's been subsequently ejaculated into a woman's mouth/throat and swallowed down her gullet.
If the jizz-burst is spat or puked out, it can no longer be called 'swallowage', it would the be called 'jizz-spat' or 'spattage'.
Chick: 'Babe, I'm really enjoying that swallowage! Hit the spot!'
Guy: 'My pleasure, chicken-tits, let me know if you're hungry again in 30 mins. or so, happy to oblige!'
Chick: 'Ok, babe, thanks for the snack!'