"large amounts," 1869, American English, earlier "dollar" (1855, usually in plural), of uncertain origin. Unknown connection to scad, the fish, which were "often very abundant and occasionally seen in enormous shoals":
In July, 1834, as Mr. Yarrell informs us, most extraordinary shoals passed up the channel along the coast of Glamorganshire; their passage occupied a week, and they were evidently in pursuit of the fry of the herring. The water appeared one dark mass of fish, and they were caught by cart-loads, and might even be baled out of the water by the hands alone. ["British Fish and Fisheries," 1849]
c.1600, Cornish name for a type of fish (also known as horse mackerel) abundant on the British coast; of uncertain origin, perhaps a variant of shad. OED compares Welsh ysgaden "herrings," Norwegian dialectal skad, Swedish skädde "flounder."