Green’s Dictionary of Slang

in the stomach, in the solar plexus, usu. relating to a blow.

1860		Chambers Jrnl 25 Aug. 117/1: [T]he Champion of England had ‘caught him right amidships’.
1886		P.D. Haywood Cruiise of Alabama 93: He gave Buster a whack amidships.
1918		Kia Ora Coo-ee 15 Oct. 4/1: During the short retirement a ‘whizz-bang’ struck the said mule amidships, and exploding, deprived the ‘annoyance’ of pack saddle and all else of military value and more or less covered the Russian with gore.
1960		Wodehouse Jeeves in the Offing 175: These words caught me amidships and took all the fighting spirit out of me.