Flat-out means completely, absolutely, or without holding anything back. When someone tells you flat-out that they won't help with your science project, they're being direct and clear: there's no maybe, no possibility of changing their mind. If you ask your friend whether they ate the last cookie and they flat-out deny it, they're stating it as pure fact.

The phrase often describes doing something with maximum effort or speed. A runner going flat-out sprints as fast as their legs will carry them, holding nothing in reserve. A race car driver pushing their vehicle flat-out uses every bit of its power.

Flat-out can also mean plainly or bluntly, especially when the truth might be uncomfortable. If your teacher says flat-out that your essay needs major work, she's being honest rather than softening the message. When someone asks a flat-out question, they want a direct answer with no dancing around the subject.

Whether describing speed, honesty, or certainty, flat-out means going all the way, with no hedging and no holding back.