Green’s Dictionary of Slang

(prizefighting) a blow to the stomach.

1825		Pierce Egan’s Life in London 3 Apr. 77/1: I] the course of the fight, he napt a stomacher that made him sick.
1831		Satirist (London) 4 Dec. 274/3: Hopplegobble received a stomacher [...] it was a twist in the bread basket.