English word and pronunciation origins are a colorful and interesting subject area, reflecting the rich history of the country. I cannot be certain of course, but the answer may be as simple as avoidance of a common vulgar expression when addressing, or referring to a military officer. A toilet in Great Britain has long been referred to as a water closet, or "loo". This nomenclature derives in turn from the old French slang expression, "Gardez l'eau"---"Look out for the water!"---which was the warning to pedestrians exclaimed by people emptying their chamber-pots onto the streets below. To the untrained English ear, "l'eau" sounds much like "lieu". Well, it just would not be in good taste or decorum to refer to a military officer as "Loo-tenant", so I suspect that "lieutenant" may have come to be pronounced "lef-tenant" to avoid the appearance of disrespect.