Green’s Dictionary of Slang

1. (UK Und.) a form of confidence trick, based on persuading a stranger that one is an old, if forgotten friend.

1828

G. Smeeton Doings in London 28: One of the modes of raising money, well known in town by the flash name of ‘Gagging,’ has been practised of late to a considerable extent on simple country-men, who are strangers to the ‘ways of town.’.

2. begging.

1913

Chelmsford Chron. 21 Feb. 7/4: The hawkers [...] did a little bit of ‘grubber’ (work) and roamed about doing a bit of ‘gagging’ (begging) .