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.

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.

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.

while banging someone in the ass managing to
successfully maneuver them around the room without bumping into any
furniture

A senior rank in the Navy.

the pitcher in the gay relationship, he navigates the "seas"

The act of engaging in sexual intercourse in the doggy style position. The giver then uses his hands to knock the recievers hands out from under them causing them to fall on their face. You are promoted to a Rear Admiral when you can push your partner anywhere in the room without using your hands.