The plimpy is a kind of fish, shaped like a ball with two long, rubbery legs and webbed feet. The freshwater plimpy is a species of the plimpy which inhabits streams and rivers. Other species live at the bottom of deep lakes. Plimpies eat snails.
If you happen to spot a plimpy with its legs tied in a knot, you will know that merpeople are around, since they consider the plimpy to be a pest and tie their legs so they drift away (FB). The stream near Bottom Bridge, behind the Lovegoods’ house, contains Freshwater Plimpies that Xenophilius uses to make soup (DH20). The Gulping Plimpy is possibly the same species as the Freshwater Plimpies, since Luna talks about warding them off with Gurdyroots (HBP20).).
"plim" Eng. a dialect word for becoming plump [NSOED]