one’s anus or asshole.
Turning this body part into an animate noun leads to two different terms – one insulting and one affectionate. The insulting term is mūxa which describes a person who says things to deliberately cause harm. These things don’t have to be lies, they just have to be intended to cause harm. So a malicious person.
The other term mūxisse is an informal term using an old diminutive and refers to people (or other dependents) one has to continually clean up after. The term expresses both affection and aggravation.