Sanskrit
अनुबन्धं क्षयं हिंसामनपेक्ष्य च पौरुषम् । मोहादारभ्यते कर्म यत्तत्तामसमुच्यते ॥ २५ ॥
anubandhaṃ kṣayaṃ hiṃsām anapekṣya ca pauruṣam mohād ārabhyate karma yat tat tāmasam ucyate
Translation
The action undertaken from delusion, without regard to capacity and consequences—loss and injury to others—that is declared to be dark.
Annie Besant · The Bhagavad-Gita, translated by Annie Besant (4th ed., 1922) — Wikisource · Public Domain
Explanation
Tamasic action is undertaken from sheer delusion, with no regard for one’s actual capacity or for the resulting loss and harm to others. This is the most consequential of the three failure modes precisely because it’s outward-facing — where rajasic action mainly costs the agent, tamasic action externalizes damage onto others through simple heedlessness.