In plain words
The agent who is uncollected, crude, stubborn, deceitful, malicious, lazy, despondent, and procrastinating is called tāmasa.
Word for word
Uncollected, unrefined, stiff, deceitful, wicked, lazy, despondent and procrastinating, the agent is called tāmasa.
अयुक्तः प्राकृतः स्तब्धः शठो नैकृतिकोऽलसःविषादी दीर्घसूत्री च कर्ता तामस उच्यते
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
अयुक्तः न युक्तः असमाहितः, प्राकृतः अत्यन्तासंस्कृतबुद्धिः बालसमः, स्तब्धः दण्डवत् न नमति कस्मैचित् , शठः मायावी शक्तिगूहनकारी, नैकृतिकः परविभेदनपरः, अलसः अप्रवृत्तिशीलः कर्तव्येष्वपि, विषादी विषादवान् सर्वदा अवसन्नस्वभावः, दीर्घसूत्री च कर्तव्यानां दीर्घप्रसारणः, सर्वदा मन्दस्वभावः, यत् अद्य श्वो वा कर्तव्यं तत् मासेनापि न करोति, यश्च एवंभूतः, सः कर्ता तामसः उच्यते ॥ २८ ॥
In plain words
'Undisciplined' means simply not disciplined, unfocused. 'Primitive' means an intellect that has never been refined at all, comparable to a child's. 'Obstinate' means stiff as a rod, bowing to no one whatsoever. 'Deceitful' means cunning, concealing his real capacities. 'Fraudulent' means intent on setting others against each other. 'Lazy' means simply inclined not to act, even in his own duties. 'Despondent' means constantly downcast by nature. And 'procrastinating' means someone who endlessly stretches out his obligations, always sluggish by nature — whatever needs doing today or tomorrow, he still hasn't done even a month later. Whoever fits this whole description is the agent called tamasic.
Close to the Sanskrit
'Undisciplined' — not disciplined, unfocused; 'primitive' — one whose intellect is utterly unrefined, comparable to a child; 'obstinate' — like a rod, he bows to no one at all; 'deceitful' — cunning, one who conceals his true capacity; 'fraudulent' — intent on causing division among others; 'lazy' — having the habit of inactivity even in his own duties; 'despondent' — possessing despondency, whose nature is always downcast; and 'procrastinating' — one who indefinitely stretches out his duties, always of sluggish nature; whatever is to be done today or tomorrow, that he does not do even in a month; and whoever is of this kind, that agent is called tamasic.