In plain words
That buddhi which, enveloped in tamas, regards adharma as dharma and sees all things in a distorted way is tāmasī, Partha.
Word for word
Which, covered by tamas, regards adharma as dharma, and all things as only contrary, that buddhi, O son of Pṛthā, is tāmasī.
अधर्मं धर्ममिति या मन्यते तमसाऽऽवृतासर्वार्थान् विपरीतांश्च बुद्धिः सा पार्थ तामसी
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
अधर्मं प्रतिषिद्धं धर्मं विहितम् इति या मन्यते जानाति तमसा आवृता सती, सर्वार्थान् सर्वानेव ज्ञेयपदार्थान् विपरीतांश्च विपरीतानेव विजानाति, बुद्धिः सा पार्थ, तामसी ॥ ३२ ॥
In plain words
The understanding so wrapped in darkness that it takes the forbidden for the right and the right for the forbidden, that looks at every single thing in front of it and sees it turned inside out — that kind of understanding, Partha, is tamasic.
Close to the Sanskrit
That understanding which — being covered over by darkness — thinks and knows unrighteousness, namely the forbidden, to be righteousness, namely the enjoined; and which perceives all objects, all knowable things indeed, as reversed, indeed as exactly reversed — that understanding, O Partha, is tamasic.