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1.
Four people engaged in sex acts at the same time. When three of the four are of the same gender, you have a gangbang. If you have two couples (two men, two women), it´s also named as round bed.
My supreme dream is to take part in a foursome.
by El Mercenario November 28, 2003
 
2.
(N) Even better than a threesome.
"Had a foursome with triplets last night."
by LarcLover October 21, 2003
 
3.
A sex act involving four people, in which everyone is engaging in sexual acts with each other. Two males and two females together in which the males are not participating in sex acts together can NOT be called a true foursome, it would be considered a swap.
Holy shit I had a foursome with two hot chicks and this one dude last night...

No way! Lucky fuckin guy.
by BentOver March 23, 2007
 
4.
Four people having sex, preferably with 3 women and 1 man, or all women.
Gregory House, M.D. "I've moved on to foursomes. That way, if one person leaves, you're still having a threesome. And if a second person leaves, you're still having sex."
by Reverend J. Izzy Commodde February 06, 2011
 
5.
My favorite type is when a female and three males have sex at the same time. This happens by Boy 1 giving Girl an anal, Boy 2 giving Girl regular sex, and Girl giving Boy 3 a blowjob.
hope this explains a foursome!!
by Cheryl_loves_UD April 10, 2013
 
6.
a four-some is when 2 couples;man & woman,man & man,woman & woman; have sex next to each other and then decide to switch partners,or they can not either works.
mike:john,his girl sarah,and this lesbien couple has a four-some!

jeff:no freakin' way!?!?!

mike:no seriously,he told me!

jeff:lucky bastard!
by leann November 23, 2006