Bhagavad Gita Chapter 18, Verse 25

अनुबन्धं क्षयं हिंसामनपेक्ष्य च पौरुषम् | मोहादारभ्यते कर्म यत्तत्तामसमुच्यते ||

anubandhaṁ kṣayaṁ hiṁsām anapekṣya ca pauruṣam mohād ārabhyate karma yat tat tāmasam ucyate

That action which is begun from delusion, without considering bondage, destruction, harm, and one's own capacity, is called tamasic.

Krishna explains tamasic action. 'Anubandhaṁ kṣayaṁ hiṁsām anapekṣya ca pauruṣam'—without considering (anapekṣya) bondage (anubandham), destruction (kṣayam), harm (hiṁsām), and one's own capacity (pauruṣam). 'Mohād ārabhyate karma yat tat tāmasam ucyate'—that action (yat karma) which is begun (ārabhyate) from delusion (mohāt) is called (ucyate) tamasic (tat tāmasam). This is tamasic action: begun from ignorance, without considering consequences—bondage, destruction, harm to others, or one's own capacity. This is the lowest kind of action: driven by tamas (ignorance), which creates delusion about what's appropriate. Unlike sattvic action (which is detached and duty-based) and rajasic action (which is passionate but aware), tamasic action is ignorant and harmful. You act without understanding consequences, without considering harm, without recognizing your capacity. This action binds you and creates suffering.