While Americans (and possibly others) pronounce this as "loo-tenant", folks from the UK pronounce it as "lef-tenant".
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Etymonline indicates that spelling with lef- dates to the 14th century, but that the origins of that spelling (and presumably its associated pronunciation) are “mysterious”. The word comes originally from Old French, and according to the OED, Old French replaced word- and syllable-final [w] with [f]; for the Modern French word lieu, this is shown by an Old French spelling variant luef. Both forms, whyever they exist, just happened to stick.
Etymology: The word lieutenant derives from French; the lieu meaning "place" as in a position; and tenant meaning "holding" as in "holding a position"
Pronunciation: Pronunciation of lieutenant is generally split between the forms /lɛfˈtɛnənt/ (About this soundlisten) lef-TEN-ənt and /luːˈtɛnənt/ (About this soundlisten) loo-TEN-ənt, with the former generally associated with the armies of British Commonwealth countries and the Republic of Ireland; and the latter generally associated with anyone from the United States or other Western Hemisphere countries. The early history of the pronunciation is unclear; Middle English spellings suggest that both proununciations may have existed even then. The majority of sixteenth- and seventeenth-century sources show pronunciations with /v/ or /f/, but Bullokar has /liu/.
The rare Old French variant spelling luef for Modern French lieu ('place') supports the suggestion that a final [u] of the Old French word was in certain environments perceived as an [f]. [Wikipedia]
I assume the 'u' / 'v' from lieu (place) just got devoiced because it precedes the voiceless /t/.
The fact that it's pronounced with an F in BrE means one of the three things as far as I can tell. Either:
In Anglo-Norman or whatever dialect of French lieutenant was borrowed from, they probably pronounced the < v >/ < u > as /v/ in some places and it took the devoicing from the /t/ here (as the 'v' in have gets devoiced when it's followed by to as in hafta).
Or, as orthographic < u > and < v > were often conflated (sometimes without exception), Anglophones for some reason hypercorrected their pronunciation to match the orthography, so: /l(j)ɛu:ˈtɛnənt/ (or /l(j)ɛwˈtɛnənt/) -> /l(j)ɛvˈtɛnənt/ -> /l(j)ɛfˈtɛnənt/.
Or, the Anglophones confused the lieu with the English word leave (or love or live, I don't know) as people often confuse V and W (v and w are allophones in some languages) and got the pronunciation /l(j)ɛvˈtɛnənt/ instead of /l(j)uːˈtɛnənt/ and then later on the /v/ got devoiced to /f/ (as it usually gets devoiced when followed by a voiceless consonant due to voicing assimilation).
After further research, I think the 'v' (which is now F) in lieutenant is epenthetic as in Modern French pouvoir (from pooir).
From Middle French povoir, from Old French povoir, pooir, from early Old French poeir, from Vulgar Latin *potēre (“to be able”), *possō for Classical Latin posse, present active infinitive of possum. The Latin infinitive *potēre was a regularized form from the root potis (“able”) or formed on the basis of the present participle potens. The v is an epenthetic consonant added to avoid hiatus. [Wikitionary]
As for the American pronunciation of lieutenant (without f), I think that's a spelling pronounciation.
Take it with a pinch of salt.
(Here's a good discussion on the topic.)
While it will always remain a mystery, I think that this goes back to the OF pronunciation of "lieu" to sound like "lyeuch". Then "lieutenant" would have the pronunciation of "lyeuchtenant". Later, when the drive by the English to rid the language of french words began, they modified the word to try to match their pronunciation and made it "leftenant". Even later, when the Americans cuddled up to the French during the revolutionary war, their pronunciation changed to follow the french term. I'll bet George Washington used the English term at the beginning.
Of course- this is opinion and I have been wrong before. It was in 1950 I believe...
The different pronunciation started sometime after England was first at war with France from 1337 to 1453. The politicians and the public wanted nothing that sounded French. They also changed German Shephard dog to Alsatian.
However, we have been at war with the Dutch and still use some Dutch expressions like Dutch courage, go Dutch, Dutch yaw and Double Dutch.
Lieu from french shows up in English phrases like "in lieu of" meaning "in place of", so lieu means place and lieutenant essentially means placeholder. There is a rarely used word, lief, which I see in old books in phrases like "I'd as lief jump off this wall as ..." and I think it is another spelling and pronunciation of lieu. I think the "lef-tenant" pronunciation comes from that lief which was no doubt interchangeable with lieu at some point in the past.
BTW Canadians technically say lef-tenant though many ordinary folks say it the US way because that's what we hear on TV.
After the post, I will try to research the etymology of 'Lieutenant.' From what I was told (Fact or fiction as it may be), in the early Naval days (circa 1600), the sword of the Commanding Officer designated who was in charge and who was their leader. The Lieutenant always had someone guarding the sword when the Commanding Officer was wearing it. The sword is carried on the left hip. That person stood to the 'left' of the Lieutenant. Therefore, he was called the Left Tenant because he was second in command to the Lieutenant. If the Lieutenant was killed, the Left Tenant took the sword and became the Lieutenant.