Quite simple really...this is a product like Depends, Attends, etc. -- any product designed to absorb micturition (piss), defecation (shit) and sometimes even menstrual discharge.

They're called, "disposable plastic adult toliets because they have but one purpose: an adult pisses, shits, and occasionally has her period (or he cums in one) when using them, they're made chiefly out plastic, and you hold your nose when you're gingerly carrying the fucking yucky, wet, heavy dripping bloody thing to the lidded wastepaperbasket or diaper pale for disposal.

A casual invitation / threat used by an individual that is not afraid of fucking someone up.