An immediate response to a stupid question, or question with a next-to-obvious answer. The placeholder is supposed to be used by itself, with the "..." implying a period of silent waiting for the question asker to re-contemplate the stupidity of his/her question and/or understand that the person that he/she is asking the question to is of superior caliber and need not waste time answering this goddam stupid question. In fact, "I mean..." need not be used after a question. A cocky fucker can just use it all the time, and not have to say anything else!
When that one slut asked DDB if he had the time, he quickly gave her that look and said, "I mean..." since a) DDB obviously doesn't have the time for some stupid slut, not to mention anyone really, and b) if she was asking for the clock time, why should he answer that when she can just look up at the sun's position in the sky, and if its night she can just wait a couple of hours, and if she's blind then why the fuck does she need to know the time anyway, I mean...