Green’s Dictionary of Slang

mumbler n.

(UK Und.) a beggar pretending to be a failed tradesman.

[UK]H.T. Potter New Dict. Cant (1795) n.p.: mumblers, and mumpers / beggars, under pretence of being decayed tradesmen.
[UK]G. Andrewes Dict. Sl. and Cant.
[UK]Flash Dict.