I'm unsure about something; I don't have a response or suggestion right now. Often used in an evasive manner when one cannot, or does not want to, answer a particular question. In informal settings, the words are commonly slurred together as "I dunno." I don't know how we can help them, I really don't. A: "Why on earth did you throw a baseball through their window?" B: "I dunno, Mom—I'll pay them for it, just leave me alone."