It has multiple definitions, a girl guys use to practice screwing with to practice for a real girl, smelly vagina's that are really wide spread and are super wet, a term used to insult someone without meaning anything directly and to insult someones girl friend with.
"Hanne is such a slosh, I knocked her in good to get ready for New Years."
"Sean: Oh my goodness, that new girl is soo cute, I wish i could go talk to her, but im too shy!
Patric: Sean, you're the biggest slosh in the universe.
Sean: What does that mean?
Patric: YOU, stupid slosh..."