In plain words
With buddhi unattached everywhere, self mastered, longing gone, one attains through Sannyāsa the supreme naiṣkarmya-siddhi.
Word for word
With buddhi unattached everywhere, mind mastered, free from longing, one attains through saṃnyāsa the supreme perfection of actionlessness.
The commentators
Bhagavadgītābhāṣyam
या कर्मजा सिद्धिः उक्ता ज्ञाननिष्ठायोग्यतालक्षणा, तस्याः फलभूता नैष्कर्म्यसिद्धिः ज्ञाननिष्ठालक्षणा च वक्तव्येति श्लोकः आरभ्यते —
असक्तबुद्धिः असक्ता सङ्गरहिता बुद्धिः अन्तःकरणं यस्य सः असक्तबुद्धिः सर्वत्र पुत्रदारादिषु आसक्तिनिमित्तेषु, जितात्मा जितः वशीकृतः आत्मा अन्तःकरणं यस्य सः जितात्मा, विगतस्पृहः विगता स्पृहा तृष्णा देहजीवितभोगेषु यस्मात् सः विगतस्पृहः, यः एवंभूतः आत्मज्ञः सः नैष्कर्म्यसिद्धिं निर्गतानि कर्माणि यस्मात् निष्क्रियब्रह्मात्मसम्बोधात् सः निष्कर्मा तस्य भावः नैष्कर्म्यम् , नैष्कर्म्यं च तत् सिद्धिश्च सा नैष्कर्म्यसिद्धिः, निष्कर्मत्वस्य वा निष्क्रियात्मरूपावस्थानलक्षणस्य सिद्धिः निष्पत्तिः, तां नैष्कर्म्यसिद्धिं परमां प्रकृष्टां कर्मजसिद्धिविलक्षणां सद्योमुक्त्यवस्थानरूपां संन्यासेन सम्यग्दर्शनेन तत्पूर्वकेण वा सर्वकर्मसंन्यासेन, अधिगच्छति प्राप्नोति । तथा च उक्तम् — ‘सर्वकर्माणि मनसा संन्यस्य नैव कुर्वन्न कारयन्नास्ते’(भ. गी. ५ । १३) इति ॥ ४९ ॥
In plain words
The attainment that comes from action has already been stated: it amounts to becoming fit for steadfastness in knowledge. And the fruit of that attainment — naiṣkarmya-siddhi, the attainment of actionlessness, which is nothing other than steadfastness in knowledge — still has to be stated. That is why the next verse begins:
'Unattached in intellect' means: his inner faculty is free of attachment toward everything — wife, children, whatever ordinarily draws a person in. 'One who has conquered himself' means: his inner faculty is brought fully under his own control. 'Free from craving' means: all thirst for body, for life, for pleasure has left him. And whoever has all three of these qualities, and also knows the Self — he reaches naiṣkarmyasiddhi, the perfection of actionlessness: because he has realized that the Self, being Brahman, was never in action to begin with, his actions simply fall away, and he is called 'actionless'; that state is called 'actionlessness'; and actionlessness together with its perfection is 'naiṣkarmyasiddhi'. Or, put differently, it is simply the attainment of abiding, steadily, in the Self's own actionless nature. He reaches that supreme, surpassing perfection — utterly unlike anything achieved through action, nothing less than living, right now, in liberation itself — through renunciation: true knowledge, or the renunciation of all action grounded in that knowledge. And so it is said: 'Having renounced all actions in the mind, one dwells at ease, neither acting nor causing others to act' (Bhagavad Gita 5.13).
Close to the Sanskrit
That attainment which is born of action has been stated, characterized by fitness for steadfastness in knowledge; and standing as the fruit of that, the attainment of actionlessness, characterized by steadfastness in knowledge, is to be stated — thus the verse is begun:
'Asaktabuddhi' means: unattached, free from attachment, is his intellect, that is inner faculty, of whom — he is 'asaktabuddhi', everywhere, that is toward wife, children, and so on, the occasions of attachment. 'Jitātmā' means: conquered, that is brought under control, is his self, that is inner faculty, of whom — he is 'jitātmā'. 'Vigataspṛha' means: departed is his craving, that is thirst, for body, life, and enjoyments, from whom — he is 'vigataspṛha'. He who, being thus qualified, is a knower of the Self — he attains naiṣkarmyasiddhi: his actions having departed because of the realization that the actionless Self is Brahman, he is 'niṣkarma'; his state is 'naiṣkarmya'; and that naiṣkarmya together with that accomplishment is 'naiṣkarmyasiddhi'. Or, it is the accomplishment, that is the attainment, of the state of actionlessness, characterized by abiding in the form of the actionless Self. That naiṣkarmyasiddhi — supreme, excellent, distinct in nature from accomplishment born of action, in the form of abiding in immediate liberation — through renunciation, that is through right vision, or through the renunciation of all action preceded by that knowledge — he attains, that is reaches. And accordingly it is said: 'Having renounced all actions mentally, one dwells, neither acting nor causing to act' (Bhagavad Gītā 5.13).