Green’s Dictionary of Slang

(UK und.) a group of apartments (usu. defined as for poor prisoners) on the top gallery of the Fleet Prison, next to the Fleet River, London.

1745		W. Paget The Humors of the Fleet 9: When paid, he puts on an important Face, / And shews Mount-scoundrel for a charming Place (q) note (q) Mount-scoundrel, so call’d from its being highly situated and belonging once to the common side, tho’ lately added to the Master’s.
1750		J. Thompson Life & Adventures 45: [A] narrow Kind of Passage , which stunk most abominably , and which I understood afterwards was called Mount Scoundrel.
1824		Smollett Peregrine Pickle in Misc. Works III 282: [T]he last comers were obliged to take up their habitation in Mount Scoundrel , an apartment most miserably furnished , in which they lay promiscuously amidst filth and vermin.
1852		G.A. Sala Twice Around the Clock 49: The man of letters is no longer supposed to write moral essays from Mount Scoundrel in the Fleet , to dine at twopenny ordinaries , and pass his leisure hours in night-cellars.
1866		Macaulay Works 521: A creature [...] perfectly qualified to decide on the comparative merits of the Common Side in the King’s Bench prison and of Mount Scoundrel in the Fleet.
1870		G.A. Sala ‘Underground London’ in Seven Frozen Sailors 427: Grub-street hacks, who knew much more about doings at Hockley-in-the-Hole or Mount Scoundrel in the Fleet than about the intrigues of St. James’s or the gossip of the coffee-houses.