[Middle English, deputy, from Old French : lieu, lieu; see lieu + tenant, present participle of tenir, to hold (from Latin tenre. See ten- in Indo-European Roots).]
Word History: What is the connection between a lieutenant governor and a lieutenant in the army? In the etymology of the word lieutenant, at least, the connection lies in their holding a place; that is, the word lieutenant is from an Old French compound made up of lieu, “place,” and tenant, “holding.” The word in Old French and the borrowed Middle English word lieutenant, first recorded near the end of the 14th century, referred to a person who acted for another as a deputy. This usage has survived, for example, in our term lieutenant governor, the deputy of the governor and the one who replaces the governor if need be. In military parlance lieutenant appears by itself as well as in compounds such as first lieutenant and second lieutenant, which muddy the water a bit, but the original notion of the word in military usage was that the officer it referred to ranked below the next one up and could replace him if need be. A lieutenant in the U.S. Army could thus step into the shoes of a captain.
