A hum drum fellow; a dull tedious narrator, a bore; also a set of gentlemen, who (Bailey says) used to meet near the Charter House, or at the King’s Head in St. John’s-street, who had more of pleasantry, and less of mystery, than the free masons.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Hum Cap * Hum DurgeonNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes