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1. adjective (sometimes hyphenated) Very busy; preoccupied with many different things at once. Primarily heard in Australia. Sorry I couldn't make it to lunch today, I've been flat chat with the kids all day. I've been so flat-chat at work that I barely have time to see my kids anymore.
2. adverb (sometimes hyphenated) At the greatest possible speed, efficiency, or power. Primarily heard in Australia. We drove that car flat chat out of town. The whole team is working flat-chat to get this project finished and perfect.