Meaning

noun bony flesh of herring-like fish usually caught during their migration to fresh water for spawning; especially of Atlantic coast

noun herring-like food fishes that migrate from the sea to fresh water to spawn

originFrom Middle English shadde, from Old English sceadd, possibly from Celtic (compare Scottish Gaelic sgadan (“herring”), Welsh ysgadan) or from Scandinavian/North Germanic (compare dialectal Norwegian skadd (“small whitefish”), Old Norse skata (“kind of fish”)), but the order of borrowing is unclear and the ultimate origin of these words is obscure.

Etymology from Wiktionary, CC BY-SA 3.0