The leftover lint from your boxers, briefs, or thongs that remain at the top of your ass crack. It is a small clump of various colors depending on the colored underwear.You know it will always be there when you take your underwear off. Higher quality underwear such as silk proves to have minimal lass effects opposed to the standard cotton underwear which is a nasty mofo. In worse cenarios if the shower does not clean it out, you may have to use your own finger to fish it out.
"My lass today was red, orange, and blue!"
"You know your day is going to suck when you have to pick your morning lass out yourself."