Green’s Dictionary of Slang

1. (US) the female genitals and pubic hair.

1981

Maledicta V 254: Fern: female genital region and pubic hair.

2. (US campus) the buttocks.

1965

B. Adler Vietnam Letters (2003) 4 Nov. 107: The hardest part of all this is the feeling of sitting around on our ferns, doing nothing, while all around us the war rages.

3. (US campus) a homosexual.

1964

in Current Sl. (1967) I:4.