v.1. To achieve a maximum value, especially as a final value: The rocket kept accelerating for a while, but finally maxed out at 6,000 miles per hour.
2. To cause something to achieve a maximum value, especially as a final value: I think I've maxed out the number of favors I can ask of my cousin. If you keep your foot on the accelerator pedal, you'll max the engine out.
3. To extend some source of credit to a maximum value or degree: I can't pay for this—I have no cash, and I maxed out my credit card. We maxed our credit line out, so we can't afford to do any more home improvements. Don't use the credit card again—we'll max out.