From the archives:

Cubital, from the Latin cubitus, elbow; also: the distance from the elbow to the fingertips (a "cubit"), an ancient unit of measurement used by the Egyptians, Greeks and Romans, among others. Depending on the time and place, it ranged between 18 and 25 inches, give or take.

Cubitus comes from cubo, Latin for "I lie down", supposedly because the ancient Romans were in the habit of resting on the forearm during meals.

Illustration by Gilbert Ford
www.jhu.edu/~jhumag/0905web/glutton2.html