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!