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."