Short for mermaphrodite. A gender which is hermaphrodite (a being with dual sex organs.) A merm has a functioning male sex organ, while the female does not. Very rarely do both work, and very rarely do neither work. The opposite of a merm is a ferm. Mermaphrodites, Fermaphrodites and Hermaphrodites are always born that way, never surgically altered, so they are not considered transexual.
She was born a mermaphrodite, but never told any of his friends.