The Collaborative International Dictionary of English v.0.48:

Brill \Brill\, n. [Cf. Corn. brilli mackerel, fr. brith
   streaked, speckled.] (Zool.)
   A fish allied to the turbot (Rhombus levis), much esteemed
   in England for food; -- called also bret, pearl, prill.
   See Bret.
   [1913 Webster]