A "rim job" is oral-anal sex. there are a lot of nerve endings near the anus, and some people find stimulating them can be enjoyable.
Of course, a lot of diseases are passed fecal-orally, meaning you can get REALLY sick from performing an unprotected rim job. Examples include hepatitis A, Shigella, E. coli, norovirus and many others.
So the proper way to perform a "rim job" is to apply lubricant to your partner's backside, cover with plastic wrap and go to town.
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