Green’s Dictionary of Slang

Pentonville Prison, London N1.

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’.
1914		E. Wittmann ‘Clipped Words’ in DN IV:ii 134: Pent (The). Pentonville Prison.