अयुक्तः प्राकृतः स्तब्धः शठो नैष्कृतिकोऽलसः। विषादी दीर्घसूत्री च कर्ता तामस उच्यते ॥

ayuktaḥ prākṛitaḥ stabdhaḥ śhaṭho naiṣhkṛitiko ‘lasaḥ viṣhādī dīrgha-sūtrī cha kartā tāmasa uchyate

Discordant, vulgar, stubborn, cheating, malicious, indolent, despairful, procrastinating, that actor is called dark.

In plain words: A dark doer is undisciplined, crude, stubborn, dishonest and spiteful. They are also lazy, gloomy and keep putting things off.