- (chiefly fandom slang) Incestuous sexual relations between twins, or siblings who look very similar.
- 1985, Joseph Hansen, A Smile in His Lifetime, New American Library, page 64:
- This one's about gay brothers. I'm calling it Twincest. Nice, no?
- 1998, Film Review: Special: Year Book 1997/98, Visual Imagination Ltd., page 117:
- As 'twincest' is in (a trend started by Parker Posey and Josh Hamilton in The House of Yes, to be released in the UK in 1998)...
- 2007, Rachel Kramer Bussel, Best Sex Writing 2008
- These pieces you might very well be able to relate to even if you've never been horny for twincest, had an affair, or been a black man...
- 2013, John Lancaster, "When did you get hooked?", London Review of Books, vol. 35, no. 7:
- In the course of one of his climbs, he hears adult voices through a high window, goes to investigate, and comes across Jaime and Cersei energetically engaged in (to use a neologism popular with fans) twincest.