Green’s Dictionary of Slang

1. (Aus.) the convict prison in Hobart, Tasmania; thus tenchman, an inmate of that prison.

1859		Broad Arrow ii 32: Prisoners’ barracks, sir – us calls it Tench [the Hobart Town Penitentiary] [F&H].
1897		‘Price Warung’ Tales of the Old Regime 143: We were all sent to a place called a tench and there we were signed off to Defferent masters.

2. Millbank Prison.

1856		H. Mayhew Great World of London II 82: The cant or thieves’ names for several London prisons or ‘sturbons’ [...] is as follows:– [...] Millbank Prison ... The ’Tench (abbr. from Penitentiary).
1857		in Punch ‘Dear Bill, This Stone-Jug’ 31 Jan. n.p.: For if Guv’ment wos here, not the Alderman’s Bench, / Newgit soon ’ud be bad as ‘the Pent,’ or ‘the Tench.’.
1862		Morn. Post 18 Dec. 3/3: He got a long turn at the Tench for the job.
1879		‘Autobiog. of a Thief’ in Macmillan’s Mag. (London) XL 503: I fell for being found in a conservatory adjoining a parlour, and got remanded at the Tench (House of Detention) for nine days.
1881		Dundee Courier (Scot.) 29 Apr. 7/3: I shall cop it hot this time. Bring my grub up to the Tench, won’t you?
1902		Globe (London) 25 Nov. 1/5: Newgate was known by its guests as ‘the Gate,’ Tothill Fields as ‘the Downs,’ and Millbank Penitentiary as ‘the Tench’ or ‘the Pen’.