cubicle n.

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).

c. 1300, "a paramour, a woman who cohabits with a man without being married to him;" also, in reference to Hebrew, Greek, Roman and other civilizations where the position was recognized by law, "a wife of inferior condition, a secondary wife," from Latin concubina (fem.), concubinus (masc.) "one who lives unmarried with a married man or woman." Usually the concubine was of a lower social order, but the institution, though below matrimonium, was less reproachful than adulterium or stuprum. The word itself is from concumbere "to lie with, to lie together, to cohabit," from assimilated form of com "with, together" (see con-) + cubare "to lie down" (see cubicle).

Such concubines were allowed by the Greek and Roman laws, and for many centuries they were more or less tolerated by the church, for both priests and laymen. The concubine of a priest was sometimes called a priestess. [Century Dictionary]

In Middle English, as in Latin, sometimes used of a man who cohabits with a woman without marriage. Related: Concubinary; concubinal.