a simile for a particularly large vagina, named so because it resembles a hippo yawning. minus the big teeth a hippo possesses of course. that would just be weird.
a hippo's yawn can usually be found attached to a slosher or perhaps a lady that has given birth to twins with heads like south park's mr mackey.
trevor : how'd it go with that slag last night then?
pete : mate, she had a minge like a hippo's yawn
trevor : that bad huh?
pete : seriously, it was like stirring a bucket of wallpaper paste with a pencil