A form of incest between twins. Twins having sexual relations.
There is often fanfic involving twins such as Fred and George Weasley having sex.

"I just read this really odd fanfiction that involved a lot of twincest!"
by epsilina August 01, 2007
sexual relations with your twin
lucy and anna were twins but they regularly ate eachother out. this is called twincest
by anonimus 1/\ November 07, 2007
The act of Incest between twins.
"I can't believe those twins are snogging! That is blatant twincest"
by Joey Masters December 07, 2007
Incest between any set of multiple birth siblings. Twincest can occur between twins, triplets, octuplets, etc. Twincest may also occur across gender lines (sisters, brothers, or both). Twincest can be any level of sexual intimacy.
The Visconti triplets practice twincest in homosexual pornography.
by T-Winner July 10, 2009
3rd person masturbation
Twins might justify twincest with each other by thinking of it as 'nothing different than masturbating from the 3rd person point of view'

Twincest between guys may often be referred to as a '3rd Person Shooter'

Mutual '3rd Person' Masturbation
by Jack U. Hoff April 28, 2008
A whole new way of 'go fuck yourself'.
When I saw those twins, all I thought of was twincest.
by HarunaXV October 10, 2010
Strong emotional and physical attraction between twins.
Did you see Tammy and Sammy together last night? Total twincest!
by powergannon July 22, 2010
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