lieu·ten·ant / loōˈtenənt/ • n. a deputy or substitute acting for a superior: two of Lenin's leading lieutenants. ∎ see first lieutenant, second lieutenant. ∎  a naval officer of a high rank, in particular a commissioned officer in the U.S. Navy or Coast Guard ranking above lieutenant junior grade and below lieutenant commander. ∎  a police or fire department officer next in rank below captain.DERIVATIVES: lieu·ten·an·cy / -ˈtenənsē/ n. (pl. -cies) .

lieutenant (arch.) vicegerent XIV; military and naval rank (orig. of one ‘holding the place’ of a captain) XVI. — (O)F., f. lieu place + tenant holder (see LIEU, TENANT). Forms with f, to which the traditional Eng. pronunc. corresponds, appear in XIV; infl. of LEAVE1 is possible.
Hence lieutenancy XV. Also lieutenant-general †vicegerent XV; rank next below a general XVI. — F. lieutenant général, in which the second word is orig. adj. in lord-lieutenant lieutenant is adj.