What does "plaice" mean?

Plaice: (a) A European food fish (Pleuronectes platessa), allied to the flounder, and growing to the weight of eight or ten pounds or more. (b) A large American flounder (Paralichthys dentatus; called also brail, puckermouth, and summer flounder. The name is sometimes applied to other allied species. [Written also plaise.] Plaice mouth, a mouth like that of a plaice; a small or wry mouth. [R.] B. Jonson.