1) When one drinks a Bloody Mary too quickly, causing the horseradish to stick to one's upper lip, creating a mustache.
2) The residue or anal hair on one's upper lip resulting from performing oral sex on one's woman from the rear.
eg 1) "Hey Mike, them boys are drinkin' your Bloody Marys so quickly they gots them some horseradish mustaches! Yee haw!"
eg 2) "Dude, had a terrible night last night. Christina sat on my face and made me go down on her. Ended up with a horseradish mustache."