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Noun. A portmanteau of the words "underwear" and "hair".

Underhair is the pubic hair that sticks out of the sides of crotch of a woman's swimsuit (esp. bikinis), when she has not shaved or waxed down there.
Jessica: Omg, Laurie...did you see Anna on the beach yesterday?
Laurie: Omg, like, totally. So embarassing...it's like she never shaved in her life. It was so thick and, like, disgusting.
Jessica: Omg, like, totally. Major underhair.
by Girl Who Cried Wolf November 22, 2013