system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why is Lieutenant pronounced "Leftenant" by british people?
gpt
(Some British people, and also all other Commonwealth armed forces, to be clear)

There is fair evidence (in terms of how the word was spelled in middle english) that both "lef" and "lieu" pronunciations have coexisted for some time.  The French word "lieu" (place) on which the word is based, is occasionally seen spelt in old French as "luef".

Codified spellings and pronunciations of words is a distinctly modern invention - for most of the time before, things were a lot more vague.