m

  1. As a numeral, M stands for one thousand, both in English and Latin.

n.

  1. 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.
  2. A brand or stigma, having the shape of an M, formerly impressed on one convicted of manslaughter and admitted to the benefit of clergy.