A senior rank in the Navy.

From an interview with "The Simpsons" creators.

Regarding "Rear Admiral," I think the answer is probably as disappointing as you feared it might be: it doesn't exist. Here is the actual first draft script excerpt from the Halloween show:

BART: Milhouse. Milhouse, wake up. Quick, look out the window.
MILHOUSE: No way, Bart. If I lean over and put my face against the window, you're gonna smash it, or maybe pinch my butt real hard.

This was the first draft. In re-writing it, the writers wanted to go for something a little funnier, something that would sound like it was from the family of "flying wedgie," "purple nurple," etc. Someone, I do not remember who, said "Rear Admiral." It sounds real, having the word "rear" in it, but it was manufactured to sound real. As far as we know, it doesn't really exist.

While performing intercourse doggystyle, the man pulls the womans hands behind her back and attempts to thrust her around a room. A full circuit around the room will make you a rear admiral.

To knee someone in the nuts from behind.

The act of engaging in intercourse with a woman from behind and using the momentum of your movement to propel you across a room. A performer of the rear admiral must hold his hands in the air so as to not balance or steady himself.

The title of Rear Admiral is given to a man that has had anal sex with 50 or more women

Rear Admiral is a slang word used for gay men in the Military...so instead of calling them 'faggot', 'homosexuals'
we call them Rear Admiral.