Source : Webster's Revised Unabridged Dictionary (1913)

Length \Length\ (l[e^]ngth), n. [OE. lengthe, AS. leng[eth], fr.
   lang, long, long; akin to D. lengte, Dan. l[ae]ngde, Sw.
   l["a]ngd, Icel. lengd. See {Long}, a. ]
   1. The longest, or longer, dimension of any object, in
      distinction from breadth or width; extent of anything from
      end to end; the longest line which can be drawn through a
      body, parallel to its sides; as, the length of a church,
      or of a ship; the length of a rope or line.

   2. A portion of space or of time considered as measured by
      its length; -- often in the plural.

            Large lengths of seas and shores.     --Shak.

            The future but a length behind the past. --Dryden.

   3. The quality or state of being long, in space or time;
      extent; duration; as, some sea birds are remarkable for
      the length of their wings; he was tired by the length of
      the sermon, and the length of his walk.

   4. A single piece or subdivision of a series, or of a number
      of long pieces which may be connected together; as, a
      length of pipe; a length of fence.

   5. Detail or amplification; unfolding; continuance as, to
      pursue a subject to a great length.

            May Heaven, great monarch, still augment your bliss
            With length of days, and every day like this.
                                                  --Dryden.

   6. Distance.[Obs.]

            He had marched to the length of Exeter. --Clarendon.

   {At length}.
      (a) At or in the full extent; without abbreviation; as,
          let the name be inserted at length.
      (b) At the end or conclusion; after a long period. See
          Syn. of At last, under {Last}.

   {At arm's length}. See under {Arm}.

Source : WordNet®

at arm's length
     adv : at some distance; "keep someone at arm's length"