When an individual licks the tip of their two fingers, straightens their mustache with said fingers and proceeds to insert these fingers inside of a vagina while licking the clitoris.
A friendly taunt of gesture made to a friend for no other reason than you can and its funny.
Put your index finger sideways under someones nose and say "mustache finger?" if you succeed and get them you will win bro points... if they somehow avoid your attempt they get bro points.