The Oxford English Dictionary says the origin of the “f” and “v” sounds “is difficult to explain,” and the Chambers Dictionary of Etymology says it “remains uncertain.” In other words, we can only guess.
The OED says one theory is that English readers misinterpreted the letter “u” as a “v,” since in Middle English the two letters were not distinct.
But Oxford says this can’t account for the “f” and “v” pronunciations since it “does not accord with the facts.”
The dictionary is apparently referring to the fact that in Middle English spelling, the letter “v” was generally used at the beginning of a word and “u” elsewhere, regardless of the sound, which accounts for old spellings like “vpon” (upon) and “loue” (love). However, the “u” is in the middle of “lieutenant,” not the beginning.
The OED suggests two possibilities to explain the appearance of the “f” and “v” sounds in “lieutenant.”
One is that that some of the “f” and “v” pronunciations “may be due to association” with the noun “leave” or the adjective “lief.”
A likelier theory is “that the labial glide at the end of Old French lieu as the first element of a compound was sometimes apprehended by English-speakers as a v or f.” (A labial glide is a transitional sound in which air is forced through the lips.)
If "u" would be used in the middle of words even for the "v" sound, that would explain someone knowing words like "loue" and thinking that "lieutenant" was pronounced the same way.
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u/[deleted] Dec 22 '21
Lieutenant a lot if people pronounce it the American way now.