Green’s Dictionary of Slang

a dive in which one lands flat on the belly (and, in extreme circumstances, winds oneself), rather than cutting through the water; also used fig.; also as adv.

1906		Vurlington Wkly Free Press 24 May 10/7: Dick heard the splash [...] ‘Gawd,’ said the captain, peering over the side, ‘Reg’lar belly-gut dive’.
1916		Tacoma Times (WA) 9 Aug. 8/6: A fleshy father essayed a dive from the board and came a ‘belly-flopper’.
1922		E. Raymond Tell England (1965) 129: Moles has gone a belly-flopper.
1929		(con. 1917–18) C. MacArthur War Bugs 187: It got so hot that we had to do belly-flops and fire from the crouch.
1939		H.E. Bates My Uncle Silas 55: It just went a belly-flopper and was done for.
1957		J. Braine Room at the Top (1959) 77: It seems to be a huge reservoir of silence into which all one’s words take belly-flops.
1959		G. Slatter Gun in My Hand 10: Heaving the stand planks into the water when we tired of bombs from the side and bellyfloppers from the board.
1962		B. McGhee Cut and Run (1963) 54: It had a frilly plunging-neckline that was doing a belly-flop.
1968		S. Gore Holy Smoke 51: You’ll come a big belly-thumper if you don’t watch out!
1971		N. Armfelt Catching Up 143: Melva, standing quivering-anxious on the brink of the pool then splatting bellyflop with an incredible smack.
1985		J. Wambaugh Secrets of Harry Bright (1986) 259: Otto [...] enjoyed himself enormously by doing belly flops.
1998		M. Anthony Midnight Come 91: Harrison’s entrance felt not so much like slipping into familiar waters as doing a violent belly flop into a [...] brackish pool.
2003		McGill Reed Dict. of N.Z. Sl.