Green’s Dictionary of Slang

1. a punch to the belly.

1821		Egan Boxiana III.
1824		Annals of Sporting 1 Mar. 199: Sending in a bellier, and rubbing down Goldie’s left lug with a chopper.
1843		Satirist & Sporting Chron. (Sydney) 4 Feb. 2/2: Baily put in two bellyers with good science.

2. a ‘belly-flop’ dive.

1981

(con. 1920s) P. Crosbie Your Dinner’s Poured Out! 133: That’s not a dive, that’s a bellier.