Why do British people say "lef‑tenant"?

Each morning, I do my word puzzles over breakfast. About 80% of the time my wife and I do them together, sometimes disagreeing on what theme the NYT connections is going for, mostly hi-fiving when we get it right and patting ourselves on the back for being so clever.

One of the games I play on my phone, Knotwords, gave me the word lieu last week, and I wondered – surely lieu and lieutenant are related? – which lead me to DuckDuckGo-ing (not as catchy as googling, but much less trackers and ads) “lieutenant etymology” and discovering that, as I suspected, lieutenant comes from the Old French lieu and tenant, meaning “place” and “to hold” respectively – literally, “placeholder”, which makes sense as the military use of the word is the “officer next in rank to the captain and commanding the company in his absence”, for which the earliest recorded use is the 1570s.

In Britain, lieutenant is often pronounced as “lef-tenant”, as opposed to the rest of the world where it is usually "lew-tenant". Some scholars have speculated that this f sound – technically speaking, a voiceless labiodental fricative – could be a result of the letters u and v being swapped or confused for each other, as in Middle English the letters could be used interchangeably. Etymonline rejects this suggestion, but offers no alternative, only stating that spellings which reflect this pronunciation date back to the fourteenth century.

In More Word Histories and Mysteries: From Aardvark to Zombie, more detail is given about variant spellings of lieutenant (spelling used to be a lot more loosey-goosey than it is now; it wasn’t until Samuel Johnson published A Dictionary of the English Language in 1755 that people started to worry more about spelling). The American Heritage Dictionaries explain that there was a variant spelling of lieu, luef, which could have been adopted by Middle English speakers, leading to the f sound. It’s also possible that, as the Oxford English Dictionary suggests, the f may have come from leave – since a lieutenant is an officer who substitutes one who is on leave.

While it seems more likely that the “lef-tenant” pronunciation came from an association with the word leave, or the variant Old French leuf, the reality is we will likely never know exactly where this funny little phonetic oddity came from. But it is fun to speculate, on cold mornings over scrambled tofu and Earl Grey.