That month in which a man’s wife lies in: wherefore, during that time, husbands plead a sort of indulgence in matters of gallantry.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Gan * GangNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes