One of the mysteries of the English language finally explained.
Definition of frost fish in US English:
frost fish
noun
1North American the tomcod, Microgadus tomcod (family Gadidae), which becomes abundant on the east coast of North America in the late autumn.
2NZ the scabbardfish, Lepidopus caudatus (family Trichiuridae), of temperate seas worldwide, known for sometimes becoming stranded ashore in the winter months.
Origin
Mid 17th century; earliest use found in William Wood (fl. 1629–1635), writer on America.