a force to reckon with

A person or thing considered to be strong, powerful, or difficult to defeat. A variant of the more common "a force to be reckoned with." They say the young boxer is a force to reckon with, so I'm betting he'll win the match tonight. The attorney may look petite and meek, but she's a force to reckon with in the courtroom. Good luck getting Stan to approve these changes—that guy's a force to reckon with.