porpoise n.

type of blunt-headed, thick-bodied cetacean common in the North Atlantic, early 14c. (late 13c. as a surname), porpas,porpays, porpeis, "the common porpoise," also the edible flesh of it, from Old French porpais (12c.) "porpoise," literally "pig fish," from porc "pig, swine" (from Latin porcus "pig," from PIE root *porko- "young pig") + peis "fish," from Latin piscis "fish" (from PIE root *pisk- "a fish").

The Old French word probably is a loan-translation of a Germanic word meaning literally "sea-hog, mere-swine;" compare Old English mereswyn, Old Norse mar-svin, Old High German meri-swin (Modern German Meerschwein), Middle Dutch mereswijn "porpoise," the last of which also was borrowed directly into French and became Modern French marsouin. Classical Latin also had a similar name, porculus marinus (in Pliny), and the notion behind the name likely is a fancied resemblance of the snout to that of a pig.