Unicorn is both literally and metaphorically a rare, magical creature that is hard to track down, but if successful, the rewards are plenty and life after that is sweet.
In the modern sense, a “unicorn” may refer to a single, bisexual woman who is willing to sleep with a straight, heterosexual couple, all the while she is not requiring any level of emotional intimacy.
A “unicorn” may also refer to the ideal partner of someone; a person that is smart, emotionally stable, empathic with a good financial basis who is also good looking and extraordinary in the bed – a rare find.
Origin of the term
The word was adopted into the English language through French in the 13th century, originating from the Latin term unicornis literally translating to “having one horn”.
Over history, the creature became the national animal of Scotland, representing strength, pride, innocence and valor.
Spread of the term
During the Victorian era, “unicorns” started to be romanticized and paired with rainbows, creating the current “fabulous” and “magical” image in our heads about the mythical creature.
A hundred years later, it was the same association that brought the “unicorn” into relation with the LGBT movement, as they chose to represent themselves with the colors of the rainbow.
Urban Dictionary provides a bountiful list of definitions on the subjects for further research: https://www.urbandictionary.com/define.php?term=Unicorn