अथ चेत्त्वमिमं धर्म्यं संग्रामं न करिष्यसि ।ततः स्वधर्मं कीर्तिं च हित्वा पापमवाप्स्यसि॥
atha cet tvam imaṃ dharmyaṃ saṅgrāmaṃ na kariṣyasitataḥ sva-dharmaṃ kīrtiṃ ca hitvā pāpam avāpsyasi
Word by word
The verse says
But if you will not fight this rightful fight, then you throw away your own work and your good name, and you take on the fault instead.
In plain words
The other side of the argument, and it is a threat.
Walk away and you lose two things: the work that was yours, and your reputation. And you do not escape fault - you acquire it.
That last part is the sting. Arjuna has been arguing that fighting is the sin. Krishna says the refusal is.
Where you've felt this · in failure
You stay out of the argument about your uncle's care so your hands stay clean. Eight months later it is obvious that staying out was itself the decision, and its results belong to you. Nobody else made them.
Going deeper
This is the first appearance of an argument the Gita will make properly in Chapter 3: inaction is not neutral.
Arjuna's entire position assumes there is an exit. Sit down, do nothing, and the guilt belongs to someone else. Krishna denies the exit exists. The war proceeds either way; the only variable is whether Arjuna is in it.
The word pāpam is usually translated sin, and that carries Christian freight it does not quite have here. Closer is fault or harm incurred - something that attaches to you as a consequence, more like a debt than a crime.
What makes the verse land is that it uses Arjuna's own accounting against him. He has been carefully calculating where fault lies. Krishna accepts the accounting and shows the sum comes out the other way.
This is still the lower register, though. It works on a man who cares about his standing, and Arjuna does. The higher answer, which cares about neither fault nor reputation, is nine verses away.