How on earth did the word "lieutenant" become pronounced "leftenant" by the english?

I understood it to be that the word comes from the French and originally meant "in the place of the holder (of the rank)". This is a very old French word and in fact "luef" is OLD French for "in place of". British English being the original form, took its pronunciation from that, and not from the (New )French term "lieu".

Thanks ren.