Green’s Dictionary of Slang

M adj.

mad.

1868		Dickens Bleak House 34: [I]forming us , with the toleration of a superior creature for the infirmities of a common mortal , that her landlord was ‘a little - M- , you know !
1877		H. Hartigan Stray Leaves (1st ser.) 183: There was only one in the troop who really thought the man ‘a little M you know’ .