Dunno/dəˈnəʊ/ Definition 1 of 1Pro Tip 1/3Informal Speech Use 'dunno' in casual conversations, not in professional or formal settings. He asked me for directions, and I just said 'dunno'. Pro Tip 2/3Spoken Usage 'Dunno' is mostly used in speaking rather than writing. Dunno if I'll go out tonight, I'm pretty tired. Pro Tip 3/3Hesitation Sound People often use 'dunno' as a filler when hesitating or thinking. Hmm, dunno, let me check.