अधर्मं धर्ममिति या मन्यते तमसाऽऽवृता। सर्वार्थान्विपरीतांश्च बुद्धिः सा पार्थ तामसी ॥

adharmaṁ dharmam iti yā manyate tamasāvṛitā sarvārthān viparītānśh cha buddhiḥ sā pārtha tāmasī

That which, enwrapped in darkness, thinketh wrong to be right and seeth all things subverted, that Reason, O Pârtha, is of darkness.

In plain words: Dark understanding is covered in ignorance. It thinks wrong is right and sees everything upside down.