And your enemies will speak many words that should not be spoken, disparaging your capacity. What could be more painful than this?

The enemies (āhitāḥ — those who wish ill) will use avācya-vādān (words not-worthy-to-be-spoken) — insults, slander. It is a violation of honor code to speak thus of an enemy, but his cowardice will invite it.

Nindantaḥ (criticizing, defaming) your sāmarthya (capacity, ability). The maximum pain (duḥkhataram) is to be dishonored by inferiors.

The question nu kim (what?) is rhetorical: there is no greater pain for the honored warrior than contempt.