Can I daisy-chain extension cords for a pressure washer?

No. Daisy-chaining two 14-gauge cords for a 10-amp motor causes a voltage drop over 100 feet that starves the motor, making it run hot and possibly damage it. The 7.5% drop exceeds the 5% manufacturers recommend, and extra connection points risk overheating. Get a single 12-gauge or 10-gauge cord instead.