Green’s Dictionary of Slang

1. an oath; thus on my davy, on my oath, on my honour; take one’s davy, to swear an oath.

1764		K. O’Hara Midas III ii: And I with my davy will back it, / I’ll swear.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Davy, I’ll take my davy of it, vulgar abbreviation of affidavit.
1811		Lex. Balatronicum [as cit. 1785].
1821		‘An Amateur’ Real Life in London II 202: It is him, and I would take my davy on it.
1833		‘Bob Dusty’ in Lummy Chaunter 73: You may all take your davy, the bucket I’ve kicked.
1833		Satirist (London) 27 Jan. 454/3: [H]e means to give in as soon as he’s took the davits!
1837		‘Nights At Sea’ in Bentley’s Misc. June 622: I’d take my daffy on it, Mister Nugent [...] I’d bet my six months’ whack again a scupper-nail that she’s a Frenchman, and a large frigate too.
1842		Boston Satirist (MA) 21 Oct. n.p.: ‘Tell me, on thy davy, whether / Thou dost dear thy Colin hold?’.
1845		J. Lindridge Sixteen-String Jack 232: I’ll be upon my davy that they had at least sixteen six-pounders, all loaded.
1848		Bell’s Life in Sydney 13 May 2/4: Miss Holland offered to take her davy it was purely platonic.
1851		Lytton My Novel (1884–5) I Bk III 176: Ay, I’d ha’ ta’en my davy on that.
1859		Hotten Dict. of Modern Sl. etc. 29: davy, ‘on my davy,’ on my affidavit, of which it is a vulgar corruption.
1860		T. Haliburton Season Ticket 295: If I was to take my davy of what I have seen when I went ashore, no livin’ soul would believe me.
1877		J. Greenwood Dick Temple III 44: I’ll take my Davy it was four [i.e. glasses] not to mention the glass of ‘neat’ you took at the bar.
1883		J. Greenwood Tag, Rag & Co. 68: This ’ere is the spot where, after they had jolly well sifted ’em, you may take your Davy – the three cart-loads of cinders and ashes [...] were thrown.
1890		H. Nisbet ‘Bail Up!’ 247: I’ll take my davy on it that that’s no old man.
1894		A. Morrison Tales of Mean Streets (1983) 71: An’ my davy, what a jore!
1896		A. Morrison Child of the Jago (1982) 114: Jist a swop for somethink else, on me solemn davy.
1902		A. Bennett Grand Babylon Hotel 96: I could have taken my dying davy that he would have been here yesterday at the latest.
1910		‘Doss Chiderdoss’ ‘A Fault Finder’ Sporting Times 15 Jan. 1/3: I’ll take my ‘davy’ / If he’d nothing to say ’gainst the sirloin of beef, / He’d be sure to find fault with the gravy!
1927		‘Jargon of the Und.’ in DN V x 443: Davy, An affadavit.
1928		V. Palmer Passage 275: I’m ready to take my solemn davy on that.
1931		Irwin Amer. Tramp and Und. Sl. 60: Davy. – An affidavit, and now seldom heard except among the older tramps.
1954		J. Phelan Tramp at Anchor 179: Tike me davy that bleeder ’ad a comb an’ slider in ’is flowery.

2. God, usu. in phrs., e.g. so help my Davy.

1864, 1867, 1870		Hotten Sl. Dict. 117: Latterly davy has become synonymous in street language with the name of the Deity; ‘so help me davy,’ slang rendering of the conclusion of the oath usually exacted of witnesses.
1873		Sl. Dict.