... that the originally spelling was "Lievtenant" (hence the sound)
but, while trying to confirm this, found the following for you:
<snip>
Lieutenant
- A subaltern officer ranking immediately below a Captain.
OED: 1578. Originally in the 14th-15th centuries, an officer acting
for a superior, or in his place. Through Elizabethan times, the
company was the largest permanently organised tactical body of
troops, and the captain's deputy was called a captain-lieutenant.
When companies grew into permanently regimented battalions, the
titles was abbreviated to simply lieutenant.
<snip>
Pronunciation: Throughout the Commonwealth the pronunciation is lef-tenant.
Possible explanations include
(a) an English interpretation of the French labial glide of lieu- as a prefix,
(b) a mispronunciation of the typographical liev-,
(c) a slur of the phrase "in lieu of".
British pronunciation may also have been influenced by the notion
that a lieutenant could not exercise power until his superior
had "left" - a confusion of the etymology with the verb "leave".
The later pronunciation loo-tenant was known in England in the late
18th century, but was never predominant, and disappeared altogether
in the 19th century. In the US, loo-tenant gained slow and intermittent
acceptance, possibly influenced by Webster's language reforms.
(Loo-tenant is a closer approximation of the original French.)
An 1893 newspaper article mentions that it was confined almost
exclusively to the retired list of the US Navy. Thirty years later
it was fast becoming the prevalent form in America.
<snip>
(See
http://www.regiments.org/milhist/regtintro/ranks.htm)
Hey, you learn something every day in this place :-)
Nihil