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. [17C–19C] a style of sexual intercourse in which the woman takes the superior position.

2. [1990s+] sexual intercourse from the rear-entry position.

In phrases

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

[mid-18C–19C] to have sexual intercourse with the woman on top of the man.