The Collaborative International Dictionary of English v.0.48:

Cubit \Cu"bit\ (k[=u]"b[i^]t), n. [L. cubitum, cubitus; elbow,
   ell, cubit, fr. (because the elbow serves for leaning upon)
   cubare to lie down, recline; cf. Gr. ky`biton elbow, ky`ptein
   to bend, stoop, kyfo`s bent, stooping, humpbacked. Cf.
   Incumbent, Covey.]
   1. (Anat.) The forearm; the ulna, a bone of the arm extending
      from elbow to wrist. [Obs.]
      [1913 Webster]

   2. A measure of length, being the distance from the elbow to
      the extremity of the middle finger.
      [1913 Webster]

   Note: The cubit varies in length in different countries, the
         Roman cubit being 17.47 inches, the Greek 18.20, the
         Hebrew somewhat longer, and the English 18 inches.
         [1913 Webster]

WordNet (r) 3.0 (2006):

cubit
    n 1: an ancient unit of length based on the length of the
         forearm

Easton's 1897 Bible Dictionary:

Cubit
   Heb. 'ammah; i.e., "mother of the arm," the fore-arm, is a word
   derived from the Latin cubitus, the lower arm. It is difficult
   to determine the exact length of this measure, from the
   uncertainty whether it included the entire length from the elbow
   to the tip of the longest finger, or only from the elbow to the
   root of the hand at the wrist. The probability is that the
   longer was the original cubit. The common computation as to the
   length of the cubit makes it 20.24 inches for the ordinary
   cubit, and 21.888 inches for the sacred one. This is the same as
   the Egyptian measurements.

     A rod or staff the measure of a cubit is called in Judg. 3:16
   _gomed_, which literally means a "cut," something "cut off." The
   LXX. and Vulgate render it "span."