Green’s Dictionary of Slang

1. strong beer or any fiery drink (cf. knock-’em-down n.).

1756		W. Toldervy Hist. of the Two Orphans III 112: Taking a dirty paper out of her bosom, in which was written the following words: Tape, glim, rushlight, white port, rasher of bacon, gunpowder, slug, wild-fire, knock-me-down, and strip-me-naked.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Knock me down, strong ale or beer, stingo.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1785].
1828		G. Smeeton Doings in London 108: In the good happy days of Old England, families sat down to breakfast on beefstakes and wholesome beer [...] off good roast beef and knock-me-down ale!
1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1866		W. Stamer Life of Adventure II 282: His father’s patronymic was the good old Irish one of Murphy, but on making a handsome fortune out of the celebrated ‘Knock me Down’ bitters [...] it was changed into the aristocratic one of De Morphie.
1873		Sl. Dict.
1900		Eve. Teleg. (Dundee) 1 Sept. 3/6: The language of the London East-end pub [...] ‘Knock me down’— very strong beer.
1903		Bulletin (Sydney) 17 Sept. 16/2: And all the trackers near and far / For his chain-lightning used to come. / It was the apple of his eye, / A fearsome home-brewed ‘Knock-me-down,’ / He used to make it on the sly / To Jew the customs folk in town.
1940		Chelmsford Chron. 26 Jan. 5/4: Old English drinks [...] Humpty-Dumpty, Hugmatee [...] Knock-Me-Down.

2. something or someone remarkable.

1858

A. Mayhew Paved with Gold 311: To see him go up a pole is a regular knock-me-down.