Green’s Dictionary of Slang

the vagina; usu. in a toast when drinking.

1650		H. Nevile Newes from the New Exchange 20: Shee [...] is excellent at the beginning of healths, viz. To the best — in Christendom, [...] for, she fits them all with a baudy Comment.
1660		Wandring Whore III 3: Lovely and ivory thighes whiter than untrod Snow, with the best red-lip’t C— in Christendom.
1673		Rochester ‘Satyr on Charles II’ in Works (1999) 85: I’ th’ isle of Britain, long since famous grown / For breeding the best Cunts in Christendom.
c.1685		York and Albany's welcome to England [broadsheet ballad] The best in Christendom, the Health, / let no Man hang an Arse:.
1684		‘The Winchester Wedding’ in Farmer Merry Songs and Ballads (1897) IV 49: But Jenkin was reckon’d for Drinking, / The best in Christendom.
1699		‘The Winchester Wedding’ in Playford Pills to Purge Melancholy I 23: But Jenkin was reckon’d for drinking, / The best in Christendom.
1700		T. Brown Amusements Serious and Comical in Works (1744) III 31: There was a shopkeeper’s wife retailing out the sight of the best in Christendom, for a half-penny a head, to young Templers, Moorfields sharpers, and old citizens that had taken the opportunity of their wives being abroad; and being ready to run mad themselves, were come to divert themselves with the sight of those that were actually so.
1709		N. Ward Compleat and Humorous Account of Remarkable Clubs (1756) 283: The first Health that is begun in the Society, is, To the best in Christendom.
1730		Life of Colonel Don Francisco 27: When the Bottle was brought up, he drank to the best in Christendom.
1734		Court Oracle 20: She fancies that she has the finest Eyes, the best best Presence, the evennest Teeth [...] and the, the, the best in Christendom.
1748		Smollett Roderick Random (1979) 348: [The glasses were] emptied in a trice, to the best in christendom.
1751		‘The Best in Christendom’ inMuse in Good Humour 213: ‘Thou who in Wisdom doth excel / relate, nor think me troublesome, / What means the — Best in Christendom? She smiled, she blushed, and with a Grace / Hung down her Head, and veil’d her face, / .
1761		‘Young D’Urfey’ ‘The Best in Christendom: a Health’ in Ways to Kill Care 88: While there’s liquor before us, / Let’s join in a chorus, / ‘To the best in all Christendom’.
1772		Bridges Burlesque Homer (3rd edn) 69: Best did I say! bar helen’s bum, / He had the best in Christendom.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Best, To the best in Christendom, i.e. the best **** in Christendom, a health formerly much in vogue.
1811		Lex. Balatronicum.