The Collaborative International Dictionary of English v.0.48:

Elbow \El"bow\, n. [AS. elboga, elnboga (akin to D. elleboga,
   OHG. elinbogo, G. ellbogen, ellenbogen, Icel. ?lnbogi; prop.;
   arm-bend); eln ell (orig., forearm) + boga a bending. See 1st
   Ell, and 4th Bow.]
   1. The joint or bend of the arm; the outer curve in the
      middle of the arm when bent.
      [1913 Webster]

            Her arms to the elbows naked.         --R. of
                                                  Gloucester.
      [1913 Webster]

   2. Any turn or bend like that of the elbow, in a wall,
      building, and the like; a sudden turn in a line of coast
      or course of a river; also, an angular or jointed part of
      any structure, as the raised arm of a chair or sofa, or a
      short pipe fitting, turning at an angle or bent.
      [1913 Webster]

   3. (Arch.) A sharp angle in any surface of wainscoting or
      other woodwork; the upright sides which flank any paneled
      work, as the sides of windows, where the jamb makes an
      elbow with the window back. --Gwilt.
      [1913 Webster]

   Note: Elbow is used adjectively or as part of a compound, to
         denote something shaped like, or acting like, an elbow;
         as, elbow joint; elbow tongs or elbow-tongs; elbowroom,
         elbow-room, or elbow room.
         [1913 Webster]

   At the elbow, very near; at hand.

   Elbow grease, energetic application of force in manual
      labor. [Low]

   Elbow in the hawse (Naut.), the twisting together of two
      cables by which a vessel rides at anchor, caused by
      swinging completely round once. --Totten.

   Elbow scissors (Surg.), scissors bent in the blade or shank
      for convenience in cutting. --Knight.

   Out at elbow, with coat worn through at the elbows; shabby;
      in needy circumstances.
      [1913 Webster]

WordNet (r) 3.0 (2006):

elbow grease
    n 1: use of physical or mental energy; hard work; "he got an A
         for effort"; "they managed only with great exertion" [syn:
         effort, elbow grease, exertion, travail, sweat]