Shad
Old English sceadd, of unknown origin.
wiktionary
From Old English sceadd, possibly from Celtic (compare Irish 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.
etymonline
shad (n.)
Old English sceadd "shad," important food fish in the Atlantic, possibly from Scandinavian (Norwegian dialectal skadd "small whitefish"); but compare Welsh ysgadan (plural), Irish and Gaelic sgadan "herring." OED says Low German schade may be from English.
Its importance suggested by its use in forming the common names of U.S. East Coast plants and wildlife whose active period coincides with the running of the shad up rivers, such as shad-bird, shad-bush, shad-flower, shad-fly, shad-frog. From the shape of the fish comes shad-bellied, 1832 in reference to persons, "having little abdominal protuberance;" of coats (1842) "sloping apart in front, cut away," especially in reference to the characteristic garb of male Quakers.
