Bhagavad Gita Chapter 18, Verse 35

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

yayā svapnaṁ bhayaṁ śokaṁ viṣādaṁ madam eva ca na vimuñcati durmedhā dhṛtiḥ sā pārtha tāmasī

O Arjuna, that firmness by which one of poor understanding does not release sleep, fear, sorrow, despondency, and delusion, is tamasic.

Krishna explains tamasic firmness. 'Yayā svapnaṁ bhayaṁ śokaṁ viṣādaṁ madam eva ca na vimuñcati durmedhā'—that firmness (yayā dhṛtiḥ) by which one of poor understanding (durmedhā) does not release (na vimuñcati) sleep (svapnam), fear (bhayam), sorrow (śokam), despondency (viṣādam), and delusion (madam). 'Dhṛtiḥ sā pārtha tāmasī'—that firmness (sā dhṛtiḥ) is tamasic (tāmasī), O Arjuna (pārtha). This is tamasic firmness: holding onto negative states—sleep, fear, sorrow, despondency, delusion—without releasing them. This is the lowest kind of firmness: stubbornly holding onto harmful states, driven by tamas (ignorance). Unlike sattvic firmness (which holds steady control through yoga) and rajasic firmness (which holds through attachment), tamasic firmness is stubbornly holding onto negative states. This is not firmness in a positive sense, but rather a stubborn refusal to release harmful patterns. This is the path: recognizing and releasing negative states rather than holding onto them.