William Smith, A Dictionary of Greek and Roman Antiquities, John Murray, London, 1875. p 357.

CONTUBERNALES (su/skhnoi). This word, in its original meaning, signified men who served in the same army and lived in the same tent. It is derived from taberna (afterwards tabernaculum), which, according to Festus, ws the original name for a military tent, as it was made of boards (tabulae). Each tent was occupied by ten soldiers (contubernales), with a subordinate officer at their head, who was called decanus, and in later times caput contubernii (Veget. De Re Mil. ii.8, 13; cf. Cic. Pro Ligar. 7; Hirt. Bell. Alex. 16; Drakenborch, Ad Liv. v.2).

Young Romans of illustrious families used to accompany a distinguished general on his expeditions, or to his province, for the purpose of gaining under his superintendence a practical training in the art of war, or in the administration of public affairs, and were, like soldiers living in the same tent, called his contubernales (Cic. Pro Coel. 30, Pro Planc. 11; Suet. Caes. 42; Tacit. Agr. 5; Frontin. Strateg. iv.1.11; Plut. Pomp. 3).

In a still wider sense, the name contubernales was applied to persons connected by ties of intimate friendship and living under the same roof (Cic. ad Fam. ix.2; Plin. Epist. ii.13); and hence when a free man and a slave, or two slaves, who were not allowed to contract a legal marriage, lived together as husband and wife, they were called contubernales; and their connection, as well as their place of residence, contubernium (Colum. xii.1.3, i.8; Petron. Sat. 96; Tac. Hist. i.43, iii.74). Cicero (ad Att. xiii.28) calls Caesar the contubernalis of Quirinus, thereby alluding to the fact that Caesar had allowed his own statue to be erected in the temple of Quirinus (cf. ad Att. xii.45, and Suet. Caes. 76).