Etymology

From Middle English mereswin ‎(“porpoise, dolphin”), from Old English mereswīn ‎(“porpoise, dolphin”), from Proto-Germanic *mariswīną ‎(“dolphin, porpoise, cetacean”), from Proto-Germanic *mari ‎(“sea”) + Proto-Germanic *swīną ‎(“swine, pig”), equivalent to mer- +‎ swine. Akin to Old High German merisuīn "dolphin, porpoise" (German Meerschwein "dolphin"), Old Norse marsvīn ‎(“dolphin, sea-pig”), Middle Low German merswīn "dolphin". More at mere, swine.