“Unicorn” describes a person who joins a couple as their third partner, for sex or even for something more committed.

What does it mean when someone says they are a unicorn?

A Unicorn is a mythical creature, someone amazing who is hard to catch or simply a very rare find. The term is often describing someone who is remarkably attractive (above a 7.9), but not at all batshit crazy, amazing at sex, and has a great personality.

What does it mean when a girl wants to be a unicorn?

People use the term unicorn because of its connotation of being a rare mythical creature. Think about it - except for a one-off threesome, it is unusual to find a woman that will have sex with a couple continually without looking for a relationship or commitment.