A platypussy is a vagina that is so incredibly tight that the penetrator literally needs to use a shoe-horn to shove his cock inside of it. Following release and withdrawal, the shoe-horn is left extruding from the cunt, resembling a hairy mammal with a duck-like beak (i.e., a platypus).
Herbert: My dear Walter, why are you having such difficulty donning your shoes?
Walter: Because I left my shoehorn inside a woman's vaginal opening last night after I shoved my chode in there.
Herbert: I dare say, it must have been a platypussy!