back-jump, n.¹ — Green’s Dictionary of Slang

back-jump n.¹

[SE back + jump n. (1c); presumably from the robber or villain’s jumping out through the entry]

a back window, spec. a prison window.

1812		Vaux Vocab. of the Flash Lang.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859, 1860		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
1882		Sydney Sl. Dict. (2 edn) 1: Back Jump - A back window (prison term).
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1895		Crowe Aus. Sl. Dict. 5: Back Jump, a back window.