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late 14c., "one who takes the place of another," from Old French lieu tenant "substitute, deputy," literally "placeholder," from lieu "place" (see lieu) + tenant, present participle of tenir "to hold" (see tenant). The notion is of a "substitute" for higher authority. Specific military sense of "officer next in rank to a captain" is from 1570s. Pronunciation with lef- is common in Britain, and spellings to reflect it date back to 14c., but the origin of this is a mystery (OED rejects suggestion that it comes from old confusion of -u- and -v-).
(only in A.V. Esther 3:12; 8:9; 9:3; Ezra 8:36), a governor or viceroy of a Persian province having both military and civil power. Correctly rendered in the Revised Version "satrap."
company grade officer, the lowest rank of commissioned officer in most armies of the world. The lieutenant normally commands a small tactical unit such as a platoon.