Green’s Dictionary of Slang

1. (UK prison) a prison sentence [one is placed inside a tub, i.e. a cell].

1890–1904

Farmer & Henley Sl. and Its Analogues.

2. an act of washing (in a bath).

1839		T. Hood Black Job n.p.: In spite of all the tubbing, rubbing, scrubbing, The routing and the grubbing, The blacks, confound them! were as black as ever [F&H].
1885		Bulletin (Sydney) 4 Jul. 14/4: No more beastly mornin’ tubbins, / No more school in Hevvin – eh, Fred? / No more sope-and-water scrubbins – / No more nothin’, now, but play, Fred!
1901		Boy’s Own Paper 11 May 498: I would very much have liked a bath [...] but ‘tubbing’ is a rare art just under the Arctic Circle.