Green’s Dictionary of Slang

euph. for God; esp. in excls. no sirree, Bob!; s’elp me bob! excl.; thus struesbob! excl.

1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 13: ‘So help me bob,’ is an oath to deceive the hearer, doubly; for a bob is but a shilling, and not a fit thing to swear by.
1894		Truth (Sydney) 13 May. 1/7: The band [...] were kicking up bob’s delight with ‘Down Went McGinty’ or something equally melodious.