I'm going to assume it's the title on whomever he's addressing, who is presumably another major character on the show, so it can't be that much of a mystery in context, surely?
Okay, so here's the 'why' of it:
- it's a title that moved into English somewhere around the 14th/15th centuries[1], from the French lieu (place) + tenant (holder - well, literally 'holding', but 'holder' when it's a noun, and obviously this is where we get the word for the opposite of a landlord, which is, interestingly enough, a good Anglo-Saxon word combination, and therefore one of the few incidents in the English language where the balance of power between the Anglo-Saxon word and its associated French word favours the Anglo-Saxon).
- It meant 'one who stands in [for his lord] in a given position/place' - so it could be a military position some way down the chain of command (metaphorically representing his lord) but more often a lieutenant literally held a place - usually a castle or similar defensive position - on behalf of his lord. (Many lords, of course, had multiple castles, and didn't spend much time in them, partly because there was a lot of other manly business to be done elsewhere like fighting and following the king around staring adoringly at him, and partly because you didn't want a huge retinue consistently staying in one place because, frankly, lots of poo. And similar other practicalities like eating all the livestock.) (Also incidentally, quite a few lieutenants of this kind - the ones that were usually set in charge of a lord's castle/manors while he was away - were the lord's wife or mother.)
- But at this point in the history of the English language the attitude to spelling was pretty much 'pffft, that is for Latin only', which is very helpful because it was phonetic (more or less) and therefore tells us a lot about pronunciation, linguistic development, regional variations and so on. And also there was a good deal of slippage between u and v: it wasn't a clear case of one is a vowel and one a consonant, just like i and j, and the difference in each scribe's hand (if there was a difference) tended to be more about their position in a word rather than their pronunciation - for example, 11 might be written 'xi' but 12 'xij' and 13 'xiij', because it's easier for the eye not to get confused when the last one has a tail on it. So at this point there was quite a bit of slippage between u and v both in writing and in pronunciation, some of which still survives in English today: eg, the u in soluble and the v in solve both come from the u/v in the Latin soluere/solvere, and the only difference is that the way we pronounced it in English meant that eventually we had to decide that one was a vowel and one a consonant.
- And of course where I'm going with this is that there was also lots of slippage in the word lieutenant, with that lovely bunch of French vowels at the front which still confuses people today and which was obviously just foreign enough to some of the people using it by then that it got spelt (and presumably pronounced) in a whole lot of more English-looking ways, like lyf- and luff- and lev-. So basically the u becomes a v which is easier to pronounce as f and the pronunciation persists because hey, we're not pronouncing it the French way anyway anymore, and also look, there's a u in it and sometimes a u is a v anyway!
- Until it occurred to somebody a few centuries later 'hey, this is wrong, there is no f in that word WHAT ARE WE DOING' and decided to go with slightly more French pronunciation. And for some reason that pronunciation caught on in at least the USA (not sure about the rest of the Anglophone Americas) and not anywhere else.
- Until American media and therefore pronunciation began to dominate the rest of the world. But that's another story.
- um. sorry. This was about Sleepy Hollow at one point. ... it looks adorable and at some point I really ought to get into watching it?
[1] The linguistic context of this is that the upper classes in England had been speaking a form of French (logically, a Norman one) since the Norman conquest (1066) but it wasn't until the 14th century that they really began a) also speaking English on a wide scale and b) spending a lot of time over on the continent fighting/associating with continental French speakers while beginning to identify as primarily English and therefore c) began to mix French words into the English language and also to notice that the French they'd been speaking for a few hundred years sounded kind of provincial and weird compared to continental forms of French, and therefore started to learn continental French as a distinct language. So, while a lot of the terminology they were using in the Hundred Years War for military matters was French-derived, for the first time that French-derived word was the English word, rather than having one word for use in French (lieutenant) and an Anglo-Saxon word that meant more or less the same for use in English.