When in a casino, you watch other players lose money playing slots. When they have had enough, you move in and take over their slot. Usually close to the jackpot the first person leave in a huff not knowing you were peering at them from behind the roulette table. Collect your winnings slot slut!!
Grandma: "Fuck, this slot machine blows!"
Sneaky person: Yes! She's giving up!
Sneaky person sits down at slot and wins. Therefore becoming an official Slot Slut.