An Enema is...

the application of a liquid into a person's rectum.

Used mainly to remove fecal matter from the anus, so that you don't gross out your partner when he takes his cock out of your ass.

Good idea too, if you like to be rimmed, so your rimming partner can enjoy getting his tongue all the way up your ass.

Common application is to use a hot water bottle, with a tube attached, that is then hung high, so that gravity will do its thing, and oh yeah, you put the tube up into your asshole.

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Gaydemon Dicktionary.