Green’s Dictionary of Slang

1. of a stage-coachman, to take on (and charge) extra passengers, without informing the coach company.

1818		W. Perry London Guide 9: Shouldering, among coachmen, is that species of cheating in which they take the fares and pocket them.
1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 158: Shouldering [...] among stage-coachmen, to shoulder, is to take up passengers on ones own account, without consulting the proprietors.
1888		Tristram Eng. lll. Mag. June 623: Shouldering in the tongue of coachmen and guards meant taking a fare not on the way-bill, and unknown to the proprietor [F&H].

2. of a servant, to cheat or embezzle from their master.

1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1882		Sydney Sl. Dict. (2 edn) 7: Shoulder - To ‘shoulder’ another by embezzling.