look up any word, like cunt:
Having sexual intercourse with 2 people at the same time
Last night i had a threesum with these to chicks
by Raynersdfgcvb June 11, 2006
Sexual intercourse between three people, usually hotter when three girls.
After hanging up on a Skype call, a threesum is usually what partakes as a nighttime activity
by iMindfreak October 30, 2011