Vagina; Possibly inspired by the alleged resmblance between a pussy and an oyster. This term, along with several others, was popularized by the Bloodhound Gang song, "Foxtrot Uniform Charlie Kilo" (F-U-C-K in Phoenetics).
*Not a term one would use in serious conversation
"Cattle prod the oyster ditch with the lap rocket" -"Foxtrot Uniform Charlie Kilo"
Jack: Did you do her up the ass?
Marc: no, but she let me have some of the oyster ditch