arm \arm\, n. as. arm, earm; akin to ohg. aram, g., d., dan., 
   sw. arm, icel. armr, goth. arms, l. armus arm, shoulder, and
   prob. to gr. ? joining, joint, shoulder, fr. the root ? to
   join, to fit together; cf. slav. rame. ?. see art,
   article.
   1. the limb of the human body which extends from the shoulder
      to the hand; also, the corresponding limb of a monkey.
      1913 webster

   2. anything resembling an arm; as,
      a the fore limb of an animal, as of a bear.
      b a limb, or locomotive or prehensile organ, of an
          invertebrate animal.
      c a branch of a tree.
      d a slender part of an instrument or machine, projecting
          from a trunk, axis, or fulcrum; as, the arm of a
          steelyard.
      e naut the end of a yard; also, the part of an anchor
          which ends in the fluke.
      f an inlet of water from the sea.
      g a support for the elbow, at the side of a chair, the
          end of a sofa, etc.
          1913 webster

   3. fig.: power; might; strength; support; as, the secular
      arm; the arm of the law.
      1913 webster

            to whom is the arm of the lord revealed? --isa. lii.
                                                  1.
      1913 webster

   arm's end, the end of the arm; a good distance off.
      --dryden.

   arm's length, the length of the arm.

   arm's reach, reach of the arm; the distance the arm can
      reach.

   to go or walk arm in arm, to go with the arm or hand
      of one linked in the arm of another. "when arm in armwe
      went along." --tennyson.

   to keep at arm's length, to keep at a distance literally
      or figuratively; not to allow to come into close contact
      or familiar intercourse.

   to work at arm's length, to work disadvantageously.
      1913 webster
see also:
art article arm's end arm's length arm's reach to go 
walk arm in arm to keep at arm's length to work at arm's length