In plain words
That firmness by which a person of poor understanding refuses to release sleep, fear, grief, depression, and the intoxication of sense indulgence is tāmasī, Partha.
Word for word
By which the person of corrupt understanding does not let go of sleep, fear, grief, dejection and intoxication too, that firmness, O son of Pṛthā, is tāmasī.
यया स्वप्नं भयं शोकं विषादं मदमेव चन विमुञ्चति दुर्मेधा धृतिः सा पार्थ तामसी
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
यया स्वप्नं निद्रां भयं त्रासं शोकं विषादं विषण्णतां मदं विषयसेवाम् आत्मनः बहुमन्यमानः मत्त इव मदम् एव च मनसि नित्यमेव कर्तव्यरूपतया कुर्वन् न विमुञ्चति धारयत्येव दुर्मेधाः कुत्सितमेधाः पुरुषः यः, तस्य धृतिः या, सा तामसी मता ॥ ३५ ॥
In plain words
That steadiness by which a dull, dim-witted person never lets go — of sleep, of slumber; of fear, of dread; of grief, of despondency, of that sunken, dejected feeling; of conceit, which is really just indulging the senses while thinking far too much of himself, like someone drunk. He keeps manufacturing all of this in his own mind, over and over, as if it were his sacred duty, and he simply will not release it — he clings. That kind of steadiness is called tamasic.
Close to the Sanskrit
That resolve by which — sleep, slumber; fear, terror; grief, despondency, the state of dejection; conceit, that is, indulgence in sense-objects, thinking oneself great, like one intoxicated, conceit indeed — these, in the mind, always, in the manner of 'this must be done', producing them, one does not release, indeed holds onto — that person of dull intellect, of contemptible intellect, who is such, whose resolve, which, that is considered tamasic.