Supposedly means a person, generally a woman, who may have slept with a generally large amount of people. Most of the time the person using this term is not actually saying it to the "slut" in question, and they normally do not actually know if the "slut" they are talking about is even a "slut" at all.
Mark, stop talking to Sandra, she's such a slut.
"Dude, you don't even know her."
"Yes, but she did sleep with James, and now she's flirting with you."
(as you can see, the two guys talking do not know her very well.)