He who blusters without reason has most reason to bluster.

A person with little to support an argument may speak with the greatest force, using noise and bravado in place of a convincing case.

The saying turns an apparent contradiction into a pointed observation: having no good reason for a claim can give someone a strong motive to sound confident.

Use it when someone responds to a challenge with loud insistence rather than an explanation. It suggests that the display of confidence may be compensating for a weak position.