pray with one’s knees upwards, v. — Green’s Dictionary of Slang

pray with one’s knees upwards v.

[i.e. in the ‘missionary position’]

of a woman, to have sexual intercourse; the implication is of promiscuity.

[1697		E. Settle The world in the moon 18: Sir Dot. Heard what Sir? Why her Prayers, (as she calls 'em) her Witches Litany, that she and her young Mephistophilus were conjuring together].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: She prays with her knees upwards; said of a woman much given to gallantry and intrigue.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.