When a teenage girl has had too much to drink, she loses all kissing abilities. Any attempt to make out with said tramp results in a sloppy mess, much like if you were to play tonsil-hockey with a largemouth bass, hence "bucketmouth."
Frat Stud: Bro, I met some fine slampiece last night.
Frat Stud 2: Did you beat it up?
Frat Stud: No, she was all over me but had the worst case of bucketmouth I've ever seen.
Frat Stud 2: That's a shame Bro, that's a shame.