lieutenantyoudaoicibaDictYouDict[lieutenant 词源字典]
lieutenant: [14] Etymologically, a lieutenant is someone who ‘holds the place’ of another (more senior) officer – that is, deputizes for him. The word comes from French lieutenant, a compound formed from lieu ‘place’ and tenant (source of English tenant). Lieu (borrowed independently by English as lieu [13] in the phrase ‘in lieu of’) comes in turn from Latin locus ‘place’, source of English local. Locum tenens [17] (or locum for short) ‘temporary replacement’, literally ‘holding the place’, is thus a parallel formation with lieutenant.

Spellings of lieutenant with -f-, indicating the still current British pronunciation /lef-/, first appear as early as the 14th century.
=> lieu, local, tenant[lieutenant etymology, lieutenant origin, 英语词源]
lieutenant (n.)youdaoicibaDictYouDict
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-).