From French 'lieu tenant' (place-holding) — literally holds a superior's place. British say 'leftenant' for unknown reasons.
A military officer of junior rank; a deputy or substitute acting for a superior; the second-in-command.
From Old French 'lieu tenant' (place-holding, one who holds the place of another), from 'lieu' (place, stead), from Latin 'locus' (place, position) + 'tenant' (holding), present participle of 'tenir' (to hold), from Latin 'tenēre' (to hold, to keep, to grasp), from PIE *ten- (to stretch, to extend, to hold). A lieutenant literally 'holds the place' of a superior — acting in their stead when they are absent. The pronunciation is one of English's great mysteries: British English says 'leftenant' while American English says 'loo-tenant.' The British form may preserve an old Anglo-Norman pronunciation where 'lieu' was
The British pronunciation 'leftenant' has puzzled linguists for centuries. The 'f' sound has no obvious source in the French spelling. The most widely accepted theory is that medieval scribes sometimes wrote 'lieu' as 'leu' or 'liev,' and the 'v' was later reinterpreted as 'f' in English (compare the words 'leaf' and 'leaves,' where 'f' and 'v' alternate). The American pronunciation 'loo-tenant' follows the French spelling more faithfully.