In plain words
But if you refuse this righteous battle, you will abandon your own dharma and honour and incur sin.
Word for word
But if you will not do this battle in accord with dharma, then, abandoning your own dharma and fame, you will incur sin.
The commentators
Bhagavadgītābhāṣyam
एवं कर्तव्यताप्राप्तमपि —
अथ चेत् त्वम् इमं धर्म्यं धर्मादनपेतं विहितं सङ्ग्रामं युद्धं न करिष्यसि चेत् , ततः तदकरणात् स्वधर्मं कीर्तिं च महादेवादिसमागमनिमित्तां हित्वा केवलं पापम् अवाप्स्यसि ॥ ३३ ॥
In plain words
So, even though it has now become a duty —
But suppose you will not fight this battle — this war that is righteous, that does not step outside dharma, that has been laid down for you — suppose you refuse. Then, by that very refusal, you throw away your own dharma and you throw away your fame, the fame that came from your meeting with Mahādeva and the others; and what you will be left holding is sin, and nothing but sin.
Close to the Sanskrit
Thus, even (that which has) arrived at the state of being what ought to be done —
But if you will not fight this righteous — not fallen away from dharma, ordained — battle, this war, if not: then, from that not-doing, having abandoned your own dharma and your fame, which had its cause in the meeting with Mahādeva and the others, you will obtain only sin.