Ch 18, Sh 46-48, Bhagawad Gita, Shankar Bhashya

Manavah, a human being; vindati, achieves; siddhim, success, merely in the form of the ability for steadfastness in Knowledge; abhyarcya, by adoring, worshipping; svakarmana, with his own duties stated above, as allotted to each caste; tam, Him, God; yatah, from whom, from which God; comes pravrttih, origin, or, from which internal Ruler comes the activities; bhutanam, of creatures, of living beings; and yena, by whom, by which God; is tatam, pervaded; sarvam, all; idam, this world.
Since this is so, therefore,
Svadharmah, one's own duty; though vigunah, defective, the word though has to be supplied; is sreyan, superior to, more praiseworthy than; paradharmat, another's duty; su-anusthitat, well performed. Kurvan, by performing; karma, a duty; svabhavaniyatam, as dictated by one's own nature, this phrase means the same as svabhavajam (born from Nature) which has been stated earlier; na apnoti, one does not incur; kilbisam, sin. As poison is not harmful to a worm born in it, so one does not incur sin by performing a duty dictated by one's own nature.
It has been said that, as in the case of a worm born in poison, a person does not incur sin while performing his duties which have been dictated by his own nature; and that someone else's duty is fraught with fear; also that, one who does not have the knowledge of the Self, (he) surely cannot remain even for a moment without doing work (cf. 3.5). Hence-
Kaunteya, O son of Kunti; na tyajet, one should not give up; what? the karma, duty; sahajam, to which one is born, which devolves from the very birth; api, even though; it be sadosam, faulty, consisting as it is of the three gunas. Hi, for; sarva-arambhah, all undertakings (whatever are begun are arambhah, i.e. 'all actions', according to the context), being constituted by the three gunas (here, the fact of being constituted by the three gunas is the cause); are avrtah, surrounded; dosena, with evil; iva, as;; agnih, fire; is dhumena, with smoke, which comes into being concurrently. One does not get freed from evil by giving up the duty to which one is born called one's own duty, even though (he may be) fulfilling sombody else's duty. Another's duty, too, is fraught with fear. The meaning is: Since action cannot be totally given up by an unenlightened person, therefore he should not relinquish it.

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Namaste