यदहङ्कारमाश्रित्य न योत्स्य इति मन्यसे। मिथ्यैष व्यवसायस्ते प्रकृतिस्त्वां नियोक्ष्यति ॥

yad ahankāram āśhritya na yotsya iti manyase mithyaiṣha vyavasāyas te prakṛitis tvāṁ niyokṣhyati

Entrenched in egoism, thou thinkest, "I will not fight;" to no purpose thy determination; nature will constrain thee.

In plain words: Krishna says that Arjuna's pride makes him think, "I will not fight." That decision will not hold, because his own nature as a warrior will push him to act anyway.