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
"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.)