AaliizahI've always thought that losei is a strange combination of lo, se, and the suffix -i, resulting in a word that roughly means "my deceitful" or "my of-deceit". This makes sense with Alduin's next line (paraphrased: You don't look like a dragon to me) and his attitude toward the Dragonborn in general (basically that s/he's a poser). I know the dialogue file translates the sentence with the word "you're" in the place of losei, but I assumed that they did this to help the actors get a general feel for what they're saying, and they may have found this word a little too complex to translate smoothly and so skimmed over it for the actor's sake. There are other cases in the dialogue file where things aren't translated literally or word-for-word, so I don't think it would really be unheard of. On the other hand, losei being a contraction of los and hi makes complete sense, especially considering what it says in the file. I don't know, I just think this should be discussed.
At first, I thought it was "los" with a mystery suffix, "-ei", which I thought to mean "you", but it still doesn't make much sense to me.