cu·bit /ˈkjubət/
  腕尺
  Cu·bit n.
  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.
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  cubit
       n : an ancient unit of length based on the length of the forearm
  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."