ancient unit of measure (usually from 18 to 22 inches) based on the forearm from elbow to fingertip, early 14c., from Latin cubitum, cubitus "the elbow, the forearm," generally regarded as a derivative of PIE *keu(b)- "to bend," but de Vaan finds this dubious based on the sense of the proposed cognates and the sound changes involved. Also compare cubicle.
It seems much safer to assume that cubitus 'elbow' is a specific instance of the ppp. cubitus of the verb cubare 'to lie down'. People lie down on their elbow if they sleep on their side, and the Romans even reclined when dining. It matters little whether the original meaning was 'forearm' or 'the elbow joint'. One may even suggest that the verb cubitare 'to lie down' ... is not (only) a frequentative to cubare, but (also) arose as a denominative 'to rest on the elbow' to cubitus. [de Vaan]
Such a measure, known by a word meaning "forearm" or the like, was known to many peoples (compare Greek pekhys, Hebrew ammah, English ell).
The word also was used in English in the "forearm, part of the arm from the elbow downward" sense (early 15c.); hence cubital "as long as a cubit" (mid-15c.), also "pertaining to the forearm" (1610s).