A brown bill, or kind of halbert, formerly carried by watchmen; corruption of MORE, great or broad, and GLAVE, blade.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Mopusses * Morning DropNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes