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1.
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
30 17
 
2.
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
5 10