Green’s Dictionary of Slang

anything especially large or notable, e.g. a lie.

1772		Bridges Burlesque Homer (3rd edn) 209: To cromeus, an East India packer, / He lent a knock, and such a whacker.
1825		C.M. Westmacott Eng. Spy I 72: First comes Marshal Thackeray, / Dress’d out in crack array; / Ar’nt he a whacker, eh?
1837		R. Nicholson Cockney Adventures 18 Nov. 19: What a whacker – here’s a sarver – crikey, what a length.
1844		J.C. Neal Peter Ploddy and Other Oddities 148: Nobody would ever tell so big a whacker as to say you are sich a one.
1857		T. Hughes Tom Brown’s School-Days (1896) 229: ‘Oh, there’s a whacker!’ cried East ‘we haven’t been within a hundred yards of his barn.’.
1861		T. Hughes Tom Brown at Oxford (1880) 333: ‘Look what whackers [...]’ said Charlie, holding out one of his prizes [...] while the indignant cray-fish flapped its tail.
1876		J. Greenwood Low-Life Deeps 289: It was a reg’ler whacker [...] One of the whackingest plum-puddin’s it was that ever was biled in a sarsepan.
1883		T.B. Reed Willoughby Captains (1887) 140: ‘Parson wanted me to do his “Caesar” for him’ ‘Oh, what a whacker!’ exclaimed Parson.
1893		Blackburn Standard 19 Aug. n.p.: When one starts to tell a cracker / It is best to tell a whacker.
1895		Crowe Aus. Sl. Dict. 94: Whacker, an extraordinary lie.
1901		Dundee Courier 24 Jan. 7/5: Here were some half-dozen other anglers waiting [...] ‘Got a whacker?’ queried one.
1906		J.W. Carr ‘Words from Northwest Arkansas’ in DN III:ii 163: whack, n. Lie. ‘Jeeminy Christmas, that’s a whack.’.
1935		G. Ingram Cockney Cavalcade 85: Blimey, ain’t he a whacker, eh?
1961		‘Frank Richards’ Billy Bunter at Butlins 100: Bunter’s postal-order must have turned up at last, and it must have been a whacker.
1970		T. Southern Blue Movie (1974) 167: Wow, that’s some whacker that guy’s got on him, isn’t it?
1999		Guardian Weekend 20 Nov. 42: Here is his German Bundesverdienst Kreuz (‘That’s a whacker!’).