Green’s Dictionary of Slang

(US) anything considered large of its kind.

1818		M.L. Weems Drunkard’s Looking Glass (1929) 77: That’s a whaler!
1833		S. Smith Major Downing (1834) 179: He’s a whaler of a fellow, as big as any two men.
1835		A.B. Longstreet Georgia Scenes (1848) 184: ‘He’s a whaler!’ said Rory; ‘but his face is mighty little for his belly and legs.’.
1865		Mississippi Clarion 17 Jan. n.p.: The captain found, on opening the ’gater’s body, two pigeons inside, whole and undigested. Oh, he was a regular waler, says the captain.
1872		Schele De Vere Americanisms 349: That the huge size of a whale should have led sailors, and after their example others also, to speak of any man or event of unusual and imposing proportions as a whaler, seems natural enough.
1907		J.W. Carr ‘Word-List from Hampstead, N.H.’ in DN III iii 204: whaler, n. Something big of its kind. ‘Was it a big calf?’ ‘Why, it was a whaler, I can tell you.’.
1944		Botkin A Treasury of Amer. Folklore 591: It were a whaler.
1954		in Randolph & Legman Ozark Folksongs and Folklore (1992) I 571: My name is Jim Taylor, my prick is a whaler, / My bollocks weigh forty-four pounds.
1965		(con. 1930s) R. Wright Lawd Today 184: My name is Jim Taylor / My john is a whaler / And my balls weigh ninety-nine pound / If you know any ladies / Who want any babies / Just tell ’em Jim Taylor’s in town.