According to LianCartman 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?
The splendid art of enjoying the flavor and scent of your womans tailpipe while you gingerly lick and sniff her funky rusty 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.