What is Brown Lips?
1.
when eating out someone's asshole, the one getting his/her ass eaten out has diarrhea into the ass eater's mouth, staining his/her lips a shitty brown color
Pete: Phil, why do your lips look like u just ate a turd?
Phil: That bitch last night gave me brown lips!
See
2.
A description for a person after giving a rimjob.
"Maggie's got some brown lips after last night."