from
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]