"animals of the mammalian order Cetacea," Old English hwæl "whale," also "walrus," from Proto-Germanic *hwalaz (source also of Old Saxon hwal, Old Norse hvalr, hvalfiskr, Swedish val, Middle Dutch wal, walvisc, Dutch walvis, Old High German wal, German Wal), from PIE *(s)kwal-o- (source also of Latin squalus "a kind of large sea fish"). In popular use it was applied to any large sea animal. Phrase whale of a "excellent or large example" is c. 1900, student slang. Whale-oil attested from mid-15c.
<a href="https://www.etymonline.com/word/whaler">Etymology of whaler by etymonline</a>
Harper, D. (n.d.). Etymology of whaler. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/whaler