“Porpoise” literally means “pig-fish” from the Old French porpais (porc “pig, swine” + peis “fish”), probably a translation of Germanic words such as the Old Norse mar-svin, meaning “mereswine,” which was also an early English word for porpoises or small dolphins.
“Porpoise” is thought to have influenced the spelling of the word tortoise, which is not from French, but from the Middle English tortuse (perhaps from Latin tartaruchus “of the underworld” or tortus “twisted.”)
As you may know, dolphins, porpoises, whales and orcas belong to the order Cetacea (from Ancient Greek kêtos, “sea-monster or huge fish”). Porpoise and dolphin are sometimes used interchangeably, but technically speaking, porpoises lack the dolphin’s long “beak” and tend to be smaller and stouter, with spade-shaped teeth and a small triangular dorsal fin. There are also almost four times as many dolphin species as porpoise species.
And on a related note, dolphin is related to the Greek delphys, meaning “womb,” suggesting the fact that it that bears live offspring or perhaps because of the animal’s shape.
Useless Etymology 