Nothing weights lighter than a promise.

The proverb is a cynical observation about how easily promises are made and how little they may ultimately count. By calling a promise “light,” it suggests that words of commitment often carry minimal real “weight” unless backed by action, cost, or accountability. The line implicitly contrasts speech with deed: a promise can be uttered with almost no effort, but fulfilling it requires follow-through, sacrifice, and reliability. As a piece of folk wisdom, it functions as a warning to be cautious about trusting assurances and, conversely, as an ethical reminder that one’s honor depends on keeping one’s word rather than merely giving it.