Twincest 

An incest between twins!

Ex:
Wow, Michael and Matthew are twincest GOALS!

Twincest by BobbySmallGland November 17, 2018

twincest fanfiction 

A slash fanfiction involving twins in a romantic and/or sexual relationship. Many fans find this an abomination.

Some Noteworthy Categories of Twincest Fanfiction:
Fred and George from Harry Potter.

Rin and Yukio from Blue Exorcist.

twincest fanfiction by SilawenGreenleaf June 29, 2011

twinsest 

like insest. but with twins

i always thought you two twins were too close. IT'S TWINSEST

twinsest by Lexxx March 23, 2007

Twinsest 

When two twins decide to have intercourse together. It also complied with triplets, quadruplets, etc.

Person 1: Do you see those red neck Twins?
Person 2: Oh yeah they Twinsest.

Twinsest by cici69 October 25, 2018

trincest 

Sex between members of Trinity College at university of Toronto. One of the most unwritten rules at the institution.

The first years are committing too much trincest during frosh week, half of cosgrave hooked up with the other half.

trincest by Trinity College September 10, 2020

Twinchester

When you motorboat twins at the same time.

John - "So how'd it go with those twins last night?"

Scott - "I gave them a twinchester."

Twinchester by estodd12 January 9, 2017

twincesses 

Twin Princesses; two girls who are just such royalty they would be considered princesses

Those girls are such twincesses.

She is my twincess for life!

twincesses by twincessprincess April 14, 2008