The very pleasurable sexual act of licking another person's anus/butthole; a fancier word for rimming, a rim job, tossing salad, analingus, or just plain eating ass.
Miranda: He kind of... licked my butt.
Samantha: Be specific. Do you mean the cheeks, or...?
Miranda: It was more localized than that.
Carrie: Wait a minute. Are we... are we talking tookus lingus?
-Sex and the City Episode 54 "Baby, Talk is Cheap"