shad n.

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. There is a late Old English sceadd, but the word seems to be missing in Middle English.

Its importance is attested 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 the 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.