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