Rimming
This is the practice of working your tongue around a sexual partner's anus. The anus is usually cleaned beforehand but not always! This produces a pleasurable experience for both parties but especially for the person being rimmed. Sometimes called "eating ass" or, less commonly, "analingus".
Rimming may indicate a special relationship between the person being rimmed (also known as the rimmee) and the person doing the rimming (also known as the rimmer).
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