lieutenant (n.) Look up lieutenant at
late 14c., "one who takes the place of another," from Old French lieu tenant "substitute, deputy," literally "place holder" (14c.), 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 "army officer next in rank to a captain and commanding the company in his absence" 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-).