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Someone who has had sex with so many people that they are virtually guaranteed to have multiple STD's. Similar to a walking STD.
Me: "Pat and John are walking petri dishes."

Jane: "I know, they're total male sluts."
by PennieGirl July 20, 2005
A girl who has sex with everyone
See that girl overthere she's a walking petri dish
by chris goggil April 21, 2007