(as) flat as a strap
1. adjective Completely flat or flattened. I left my soda open overnight and now it's flat as a strap. The roads in this part of the country are as flat as a strap—no hills, no dips, nothing!
2. adjective Extremely fast. Primarily heard in Australia. John's new car is a beauty, and I bet it's flat as a strap.
3. adjective 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 as flat as a strap with the kids all day. I've been flat as a strap at work lately—I barely have time to see my kids anymore.
4. adverb At the greatest possible speed, efficiency, or power. Primarily heard in Australia. We drove that car flat as a strap out of town. The whole team is working as flat as a strap to get this project finished on time.