The Collaborative International Dictionary of English v.0.48:

M \M\, n.
   1. (Print.) A quadrat, the face or top of which is a perfect
      square; also, the size of such a square in any given size
      of type, used as the unit of measurement for that type:
      500 m's of pica would be a piece of matter whose length
      and breadth in pica m's multiplied together produce that
      number. [Written also em.]
      [1913 Webster]

   2. (law) A brand or stigma, having the shape of an M,
      formerly impressed on one convicted of manslaughter and
      admitted to the benefit of clergy.
      [1913 Webster]

   M roof (Arch.), a kind of roof formed by the junction of
      two common roofs with a valley between them, so that the
      section resembles the letter M.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

M \M\ ([e^]m).
   1. M, the thirteenth letter of the English alphabet, is a
      vocal consonant, and from the manner of its formation, is
      called the labio-nasal consonant. See Guide to
      Pronunciation, [sect][sect] 178-180, 242.
      [1913 Webster]

   Note: The letter M came into English from the Greek, through
         the Latin, the form of the Greek letter being further
         derived from the Ph[oe]nician, and ultimately, it is
         believed, from the Egyptian. Etymologically M is
         related to n, in lime, linden; emmet, ant; also to b.
         [1913 Webster] M is readily followed by b and p. the
         position of the lips in the formation of both letters
         being the same. The relation of b and m is the same as
         that of d and t to n. and that of g and k to ng.
         [1913 Webster]

   2. As a numeral, M stands for one thousand, both in English
      and Latin.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

M. When persons were convicted of manslaughter in England, they were
formerly marked with this letter on the brawn of the thumb.
     2. This letter is sometimes put on the face of treasury notes of the
United States, and signifies that the treasury note bears interest at the
rate of one mill per centum, and not one per centum interest. 13 Peters,
176.