Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

   3. Fig.: Power; might; strength; support; as, the secular
      arm; the arm of the law.

            To whom is the arm of the Lord revealed? --Isa. lii.
                                                  1.

   {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.

Arms \Arms\, n. pl. [OE. armes, F. arme, pl. armes, fr. L. arma,
   pl., arms, orig. fittings, akin to armus shoulder, and E.
   arm. See {Arm}, n.]
   1. Instruments or weapons of offense or defense.

            He lays down his arms, but not his wiles. --Milton.

            Three horses and three goodly suits of arms.
                                                  --Tennyson.

   2. The deeds or exploits of war; military service or science.
      ``Arms and the man I sing.'' --Dryden.

   3. (Law) Anything which a man takes in his hand in anger, to
      strike or assault another with; an aggressive weapon.
      --Cowell. Blackstone.

   4. (Her.) The ensigns armorial of a family, consisting of
      figures and colors borne in shields, banners, etc., as
      marks of dignity and distinction, and descending from
      father to son.

   5. (Falconry) The legs of a hawk from the thigh to the foot.
      --Halliwell.

   {Bred to arms}, educated to the profession of a soldier.

   {In arms}, armed for war; in a state of hostility.

   {Small arms}, portable firearms known as muskets, rifles,
      carbines, pistols, etc.

   {A stand of arms}, a complete set for one soldier, as a
      musket, bayonet, cartridge box and belt; frequently, the
      musket and bayonet alone.

   {To arms}! a summons to war or battle.

   {Under arms}, armed and equipped and in readiness for battle,
      or for a military parade.

   {Arm's end},

   {Arm's length},

   {Arm's reach}. See under {Arm}.

Source : WordNet®

arm's length
     n : a distance sufficient to exclude intimacy