An unfinished sentence
with an implied
ending (i.e. the rest of the sentence would be something like "you wouldn't understand anyhow"
). Usually when people say this, the implicit
"I realize that you just asked a question, but the answer to that question is so obvious that even if I tell you the answer, you will probably be too stupid to understand it."
Sometimes it is said with a little more humorous undertones:
"I have known you for a while (or, You seem like a sharp fellow), so I'm sure that you don't realize how stupid that question is. Surely you really know the answer and just slipped up. If I smile while I say this, maybe you'll say something like 'heh heh how silly of me, of course', and then I won't have to publicly humiliate you."