A girl or guy that has a mild to moderate mental retardation when selecting their multiple sex partners.
A down syndrome slut can be sarcastically used to make fun of regular sluts who make bad decisions involving their sex partners, but it can also apply to medically diagnosed down syndrome people who are primiscuous.
"I can't believe Annie slept with that ugly transexual!" "Anyone would think she's turning into a down syndrome slut!"
"Don't tell me who and who I can't fuck you down syndrome slut!"