Lieutenant is derived from the French
lieu tenente, 'place-holder', being originally a place-holder for the
Captain - ie. someone who kept his
files in order, brought him
cups of tea, and all sorts of other such stuff. Hence the
Royal Navy has the right idea on the pronunciation: "L'tenant" being the nearest
anglicised version of the original French.
Other notable pronunciations (wrong, but we can let them off) are:
The British Army and RAF say "Lef-tenant";
Americans say "loo-tenant", which is how it looks, but we're not concerned here with form over function.