Green’s Dictionary of Slang

(US) a brothel madame.

1851		Manchester Spy (NH) 15 Nov. n.p.: Indignant at the insult [...] he repaired to Marm Lock’s.
1856		Life in Boston & N.Y. (Boston, MA) 11 Oct. n.p.: Do the people who live in the house with Marm Godfrey [...] know what an old wanton she is.
1860		N.E. Police Gaz. (Boston, MA) 18 Aug. 5/1: Does Marm H., of Warren street, consider our notice of last week as a good advertisement.