Sanskrit
यया स्वप्नं भयं शोकं विषादं मदमेव च । न विमुञ्चति दुर्मेधा धृतिः सा पार्थ तामसी ॥ ३५ ॥
yayā svapnaṃ bhayaṃ śokaṃ viṣādaṃ madam eva ca na vimuñcati durmedhā dhṛtiḥ sā pārtha tāmasī
Translation
That by which one from stupidity doth not abandon sleep, fear, grief, despair, and also vanity, that firmness, O Partha, is dark.
Annie Besant · The Bhagavad-Gita, translated by Annie Besant (4th ed., 1922) — Wikisource · Public Domain
Explanation
Tamasic dhriti is the stubbornness of stupidity — refusing to let go of sleep, fear, grief, despair, and vanity. Unlike rajasic firmness, which at least grips something worth naming, duty, desire, wealth, tamasic firmness grips nothing worth having; it’s tenacity in service of inertia rather than of any goal at all.