The usually red stain on the upper lip from Kool Aid that resembles a moustache. It usually is shaped in a semi-circle fashion due to the shape of the cup and looks like the Pringles Guy's moustache. While it is acceptable on children if a person over 11 has one they should live in complete shame.
Kenny: Dude is that a Kool Aid Moustache on your lip Alex?
Alex: No, its just a mark I got earlier.
Kenny: Bullshit, your 20 years old and you still haven't learned to drink from a glass you dumbass