Originating in Uruguay, "Milk Vampire" is a term for a person (usually a young female) who has a thirst for semen. The Milk Vampire will stalk out his/her male prey carefully, using sly methods to trap the victim, such as hiding under tables and striking any nearby male genitalia.
The Milk Vampire doesn't bite; she sucks.
A baby who breastfeeds.
My wife's friend just had twins; she now has two milk vampires to feed all day. Brutal!