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 strap with the kids all day. I've been so flat-strap at work that I barely have time to see my kids anymore.
2. (sometimes hyphenated) At the greatest possible speed, efficiency, or power. Primarily heard in Australia. We drove that car flat strap out of town. The whole team is working flat-strap to get this project finished on time.