Brown Smile

is a sexual act in which one partner licks and eats out the others asshole, to the point that when the one partner smiles there is shit in between their teeth, causing the brown smile. (Please don't let this happen to you, always make sure to thoroughly wash beforehand)

Dude1: Yo dude last night my girl did the weirdest thing.
Dude2: What?
Dude1: She licked my asshole.
Dude2: Ewwww, bro.... she gave you a brown smile!
Dude1: ....Yup