In plain words
Your enemies will speak many unworthy words, mocking your strength. What could be more painful than that?
Word for word
And your enemies will speak many words not to be spoken, deriding your prowess; what, pray, is more painful than that?
The commentators
Bhagavadgītābhāṣyam
किञ्च—
अवाच्यवादान् अवक्तव्यवादांश्च बहून् अनेकप्रकारान् वदिष्यन्ति तव अहिताः शत्रवः निन्दन्तः कुत्सयन्तः तव त्वदीयं सामर्थ्यं निवातकवचादियुद्धनिमित्तम् । ततः तस्मात् निन्दाप्राप्तेर्दुःखात् दुःखतरं नु किम् , ततः कष्टतरं दुःखं नास्तीत्यर्थः ॥ ३६ ॥
In plain words
There is more—
Words no one should ever have to say — words unfit for the mouth — and plenty of them, in every variety: that is what your ill-wishers, meaning your enemies, will be saying. Censuring you, which is to say sneering at your strength, your own strength, the strength everyone knows from the fight with the Nivātakavacas and battles like it. And than that — than the pain of having that disgrace land on you — what is worse? Nothing. That is the sense: no grief is heavier than this one.
Close to the Sanskrit
And what more—
Words not to be spoken — that is, words unfit to be uttered — many, of many kinds, your ill-wishers, namely your enemies, will speak, censuring, that is, reviling, your — your own — capacity, whose ground is the battle with the Nivātakavacas and the like. Than that, that is, than that pain of incurring censure, what indeed is more painful? The meaning is: there is no grief more grievous than that.