Green’s Dictionary of Slang

a prison warder or turnkey.

1812		Vaux Vocab. of the Flash Lang.
1828		‘The Song of the Young Prig’ in C. Hindley James Catnach (1878) 172: But my rum chants ne’er fail, sirs, / The dubsman’s senses to engage.
1834		(con. 1737–9) W.H. Ainsworth Rookwood (1857) 179: But I slipped my darbies one morn in May, [...] And gave to the dubsman a holiday.
1841		‘Bon Gaultier’ ‘The Nutty Blowen’ in Farmer Musa Pedestris (1896) 126: The dubsman’s ruthless hand has cropped her once luxurious hair.
1859, 1860		Hotten Dict. of Modern Sl. etc.
1873		Sl. Dict.