Green’s Dictionary of Slang

St George n.

also George-a-horseback
[illustrations of a mounted St George slaying the Dragon; note Lat. use, translated as ‘mounting the Hectorean horse’]

1. a style of sexual intercourse in which the woman takes the superior position.

[[UK]Fletcher Mad Lover I i: How our St. Georges will bestride the dragons, The red and ramping dragons!].
[UK]Massinger & Field Fatal Dowry (1632) III i: You did not see ’em on my Couch within, Like George-a-horsebacke, on her, nor a-bed?
[UK]J. Phillips Maronides (1678) VI 111: St George he takes a furious course, / The Dragon spits, away flies horse, / Leaving St George upon the grass, / The sport of many a pretty Lass.
[UK]T. Rowlandson Pretty Little Games (1872) plate i: Across his legs the nymph he takes, / And with St. George a motion makes / She ever ready in her way / His pike of pleasure keeps in play.
[UK] ‘Sub-Umbra, or Sport among the She-Noodles’ in Pearl 4 Oct. 2: Annie, straddling over me, and putting away my excited engine of love up her own longing crack, and beginning a delightful St. George.
[UK]Randiana 64: I suggested her mounting me, a la St. George [...] I have always been distinguished as being particularly au fait at the St George, so I managed to roll over very gradually [...] till I had got Fanny fairly planted on top of me.
[UK]C. Deveureux Venus in India I 56: I was just going to purpose that she should put her thighs outside mine, and let me have her a la St. George.
[UK]Bride’s Letter in Bold (1979) 40: So I mounted at once and, as something to brag on, / Enacted the part of St George and the dragon.
[UK]Forbidden Fruit n.p.: Come on, girls, for a wind up. Selina shall be St. George, as I lay back on the couch and suck Gert’s delicious Cunt.

2. sexual intercourse from the rear-entry position; note mis-defined as a v. in cit. 1999.

[UK]Roger’s Profanisaurus in Viz 98 Oct. 26: St. Georgev. To lance your old dragon from behind. A dog’s marriage (qv).

In phrases

ride (a) St George (v.) (also mount St George)

to have sexual intercourse with the woman on top of the man; cit. 1691 (and the whole ballad from which it comes) would appear to be an extended sexual metaphor/double entendre.

[[UK]‘The Hunting’ in Ebsworth Merry Drollery Compleat (1875) 39: Ride, ride, St. George, he’s stole into the bush].
‘Windsor Ballad’ Windsor Medley 19 : The Story’s true, I do not forge, / Each Lass at Night must mount St. GEORGE.
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: [addendum to prev.] ...this is said to be the Way to get a Bishop.
[UK]Harris’s List of Covent-Garden Ladies 49: She [...] loves to be uppermost [...] as she cannot only tame the dragon but even ride St George.
[UK]Lex. Balatronicum.
[UK]Sl. Dict.
[UK] ‘Lady Pokingham’ in Pearl 10 Apr. 33: He even once contrived to get his long thin prick into my cunt alongside of James’s as I was riding a St. George.