Source : Webster's Revised Unabridged Dictionary (1913)

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.]

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

   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.

Source : WordNet®

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