Green’s Dictionary of Slang

1. (Aus.) to use a horse without its owner’s knowledge or permission.

1904

Sydney Sportsman (Surry Hills, NSW) 23 Mar. 4/1: ‘Soldiering' a neighbor’s horse is considered in some parts more venial than treating his wife to a like operation.

2. (UK black) to act in a soldierly manner, i.e. brave, aggressive, violent.

2020

G. Krauze What They Was 139: You gotta ride it [i.e. prison] out, you gotta soldier that shit.

3. see come the old soldier under come the... v.