Green’s Dictionary of Slang

to enter a house by a back entrance, either a door or window; thus back-jumper, a thief who enters houses via a back door or window.

1891		F.W. Carew Autobiog. of a Gipsey 416: It ended by my doin’ little snakesman for my nibs and back-jumpin’ the carsey.
1901		Argus (Melbourne) 20 Sept. 6/4: Going through the back window is described as the back jump, or going the jump .