Green’s Dictionary of Slang

a knock-down blow.

1812		Egan Boxiana I 25: Bob now punished him in fine style, drove the Venetian all over the stage, and soon gave him a leveller.
1853		N.Y. Clipper 10 Sept. 1/4: Cribb received a leveller in consequence of his distance being incorrect.
1855		(con. 1810) Fights for the Championship 45: Cribb received a leveller.
1859		Matsell Vocabulum 127: leveller. When one of the contestants is brought completely to the ground.
1895		C. Crowe Aus. Sl. Dict. 45: Leveller, when by a blow one of the contestants fall in the ring.