Green’s Dictionary of Slang

to throw oneself down; to collapse, to fail badly.

[1914		B.J. Brookes diary 31 Aug. 🌐 Until now I had been doing Company work, Belly flopping (i.e. extended order drill) guards, digging, fatigues etc.].
1929		(con. 1917–18) C. MacArthur War Bugs 114: It was fun to belly-flop on the crest and watch the battle.
1961		(con. early 1950s) J. Peacock Valhalla 299: No one yelled ‘Mine!’ and chortling happily belly-flopped on it.
1980		M. Bail Homesickness (1999) 180: The red-haired bosun poised on the conning tower dived off, bellyflopping.
2000		W. Shaw Westsiders 18: The independent label that was going to release his CD belly-flopped.