It is meant to signify that she "soaks up semen" from multiple males, and that this is her prime function in life.
Cam Neely: She's got a sweet ass, but she's a real jizzrag.
Dave: I'd better go get an HIV test then.
Cam Neely: you're gonna have to get one in 3 months too.
called so because of its moistened nature from the male owner's over-excitement.