The word 'lieutenant' means '(one) holding the place of (another)' from the French root-words. In Old French, both 'lieu' and 'luef' were used to mean 'place', the latter more rarely. English seems to have adopted the SOUND of one of the old words and the SPELLING of the other, whereas Americans etc ended up with the matching pair, with the spelling as extended, more or less, into modern French. Certainly, the Scottish poet, John Barbour, wrote of a 'luftenand' as long ago as the mid 14th century.