According to Lian Cartman from South Park, it's when you contort yourself into a pretzel and allow someone to give you oral pleasure in, on and around your sphincter.
SHEILA: What did my son say Mr. Mackie? Did he say the S-woid?
MR. MACKIE: No, it was worse than that.
SHEILA: The F-woid?
MR. MACKIE: Well here’s a short list of the things your kids've been saying, m’kay?
SHARON MARSH: Oh dear God!
SHEILA BROFLOVSKY: What the heck is a "rimjob"?
LIAN CARTMAN: Oh, why that’s when you put your legs behind your head and have someone lick your ass!
The splendid art of enjoying the flavor and scent of your womans tailpipe while you gingerly lick and sniff her funkyrusty sheriffs star. All this culminating in a total mouth to tailhole suction and licking resulting in a thorough cleansing and an anal orgasm for her.
There's nothing better than giving my old lady a rimjob before I eat her pussy out. After that she'll usually suck my dick.