Military jargon: Lieutenant.
If written as a prefix to a name, it's read aloud as "lieutenant," denoting rank.
If it's pronounced "Ell Tee," it's a nickname, and it has a lot of the connotations of the word "jefe" in Spanish. In its naming form, it almost definitely refers to a second lieutenant, and is more properly written "L.T." Since a ranking officer can call a lieutenant by his or her first name, this second form is most often used by enlisted personnel. It can denote any of the following:
- "Hey L.T., let's take that hill!"
- Familiarity, born of respect and admiration - like calling your dad "Pops" instead of "sir".
- "Good morning, L.T., how are ya doing?"
- Friendly familiarity with a stranger - if you sit down on the plane next to a man dressed as a doctor, you might call him "doc", because without knowing his last name, just calling him "Doctor" sounds awkward.
- "The new L.T. says we need to get this place cleaned up for the Colonel."
- Useful short form - like referring to your boss as "the boss" instead of "Mr. Hoffstadtler" when he's not around.
Unless you're part of a mixed-rank flight crew, you will probably never hear an enlisted person refer to a lieutenant as anything more familiar than LT.