Green’s Dictionary of Slang

lying on one’s stomach.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Bung Upwards, said of a person lying on their face.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.: Bung Upwards. Said of a person lying on his face.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.