The Collaborative International Dictionary of English v.0.48:

Mainor \Main"or\, n. [Anglo-Norm. meinoure, OF. manuevre. See
   Maneuver.] (O. Eng. Law)
   A thing stolen found on the person of the thief.
   [1913 Webster]

   Note: A thief was said to be "taken with the mainor," when he
         was taken with the thing stolen upon him, that is, in
         his hands. --Wharton. --Bouvier.
         [1913 Webster]