यया स्वप्नं भयं शोकं विषादं मदमेव च। न विमुञ्चति दुर्मेधा धृतिः सा पार्थ तामसी ॥

yayā svapnaṁ bhayaṁ śhokaṁ viṣhādaṁ madam eva cha na vimuñchati durmedhā dhṛitiḥ sā pārtha tāmasī

That by which one from stupidity doth not abandon sleep, fear, grief, despair, and also vanity, that firmness, O Pârtha, is dark.

In plain words: Dark firmness is stubbornly holding on to too much sleep, fear, grief, gloom and pride, out of foolishness.