A collective noun. These are groups of chicks who travel together on Friday and Saturday nights, walking into clubs and bars after 11pm like cowboys in the old spaghetti westerns.
They are looking for a drink and a fuck.
The shame of it is they are pig ugly harridans and what no sane dude is going to go for.
Dude 1: "Hey, Dawg, check out those three ugly chicks what just walked in!"
Dude 2: "Yeeech! The Unfuckables!"