āvṛtaḿ jñānam etena
jñānino nitya-vairiṇā
kāma-rūpeṇa kaunteya
duṣpūreṇānalena ca

Translation of Bhagavad Gita 3.39

Thus the wise living entity’s pure consciousness becomes covered by his eternal enemy in the form of lust, which is never satisfied and which burns like fire.

Commentary by Sri A.C. Bhaktivedanta Swami Prabhupada of Gaudiya Sampradaya:

It is said in the Manu-smriti that lust cannot be satisfied by any amount of sense enjoyment, just as fire is never extinguished by a constant supply of fuel. In the material world, the center of all activities is sex, and thus this material world is called maithunya-agara, or the shackles of sex life. In the ordinary prison house, criminals are kept within bars; similarly, the criminals who are disobedient to the laws of the Lord are shackled by sex life. Advancement of material civilization on the basis of sense gratification means increasing the duration of the material existence of a living entity. Therefore, this lust is the symbol of ignorance by which the living entity is kept within the material world. While one enjoys sense gratification, it may be that there is some feeling of happiness, but actually that so-called feeling of happiness is the ultimate enemy of the sense enjoyer.

Commentary by Sri Vishvanatha Chakravarthi Thakur of Gaudiya Sampradaya:

This verse explains that lust is indeed ignorance of the jiva, for it covers the knowledge of the jiva. The phrase “eternal enemy” indicates that lust must by all means be killed. By this ignorance in the form of lust, compared to an unsatisfied fire, knowledge is covered. Ca means like in this sentence. It is said:

na jatu kamah kamanam upabhogena samyati
havisa krsna-vartmeva bhuya evabhivardhate

As supplying butter to a fire does not diminish the fire but instead increases it more and more, the endeavor to stop lusty desires by continual enjoyment can never be successful. SB 9.19.14

Commentary by Sri Ramanuja of Sri Sampradaya:

3.39 The knowledge, having the self for its subject, of this embodied person (the Jiva) whose nature is knowledge, is enveloped by this constant enemy in the shape of desire, which brings about attachment for sense-objects. This desire is difficult to satisfy, i.e., has for its object things unworthy of attainment and is insatiable, i.e., never attains satisfaction. Now listen to what constitutes the instruments with which desire subdues the self. Sri Krsna goes on to expound this:

Commentary by Sri Sridhara Swami of Rudra Sampradaya:

Now the inimical nature of kama or lust is being made even more explicit by Lord Krishna. It is the discriminative faculty of knowledge that is covered by kama. To the ignorant while enjoying sense objects kama itself is the cause of pleasure; but the subsequent reaction will be inimical. To the person situated in Vedic wisdom who understands that they will receive a subsequent reaction, then kama is a cause of pain even while trying to enjoy. Therefore kama is understood to be an eternal enemy. Moreover even if supplied with delightful sense objects of enjoyment it is never enough because kama is insatiable and when it is not satisfied kama turns to krodha or anger and that leads to grief and affliction just like a forest fire burning everything in its path. So from these statements its eternal inimical nature towards all beings has been declared.

Commentary by Sri Madhvacharya of Brahma Sampradaya:

One on the path of jnana yoga knows from the Vedic scriptures that moksa or liberation from the cycle of birth and death can never manifest itself until first attaining atma-tattva or soul realisation. When such is the case for persons of wisdom what can be said for persons of meagre intelligence. The word kama-rupena means in the form of lust. This indicates that only with great pain and difficulty is lust ever fulfilled. The word duspurena means never satisfied. This indicates that kama or lust is an eternal adversary. Even the position of Indra in the heavenly realms is born of this kama that is to be the king of all the celestials; yet even while trying to achieve the position of Indra, the position of Brahma is sought because kama is never satisfied and it is more exalted to be the ruler of the material worlds then the king of the celestials and so forth and so on. The word analena means like a fire which denotes the burning fire of kama which is never satisfied and insatiable. It is said that kama covers the atma as smoke covers fire, As dust prevents one from seeing a reflection in the mirror this kama prevents one from perceiving their atma and as the womb imprisons the embryo it is kama which keeps a person imprisoned in the material existence. This is Lord Krishna’s meaning.

Commentary by Sri Keshava Kashmiri of Kumara Sampradaya:

Here Lord Krishna illustrates the inimical nature of kama or lust with three lucid examples concluding that discriminatory knowledge of even basic right and wrong and even common sense is completely neutralised by the influence of kama causing one to act in base and degraded ways. The ignorant though are always pursuing kama because they derive pleasure while enjoying the sense objects afterwards when they feel the pain of the reaction they are forced to accept they may see kama as an enemy. The person situated in Vedic wisdom sees kama as an enemy from the beginning even while contemplating enjoying the action and later if they actually consumate the action then they know a painful reaction is forthcoming. Thus for those of Vedic wisdom kama is known as being an eternal enemy.

Commentary by Sri Adi Shankaracharya of Advaita Sampradaya:

3.39 Jnanam, Knowledge; is avrtam, covered; etena, by this; nityavairina, constant enemy; jnaninah, of the wise. For the wise person knows even earlier, ‘I am being induced by this into evil.’ And he always [Both at the time when desire arises in him, and also when he is forced to act by it.] feels distressed. Therefore, it is the constant enemy of the wise but not of a fool. For the fool looks upon desire as a friend so long as hankering lasts. When sorrow comes as a consequence, he realizes, ‘I have been driven into sorrow because of longings’, but certainly not earlier. Therefore it is the constant enemy of the wise alone. In what form? Kama-rupena, in the form of desire-tha which has wish itself as its expression is kama-rupa; in that form-; (and) duspurena, which is an insatiable; analena, fire. That which is difficult to satisfy is duspurah; and (derivatively) that which never has enough (alam) is analam. Again, having what as its abode does desire, in the form of a viel over Knowledge, become the enemy of all? Since when the abode of an enemy is known, it is possible to easily slay the enemy, therefore the Lord says:

Commentary by Sri Abhinavagupta of Kaula Tantra Sampradaya:

3.39 Avrtam etc. Looks like a desired one : For it acts when there is desire. It is fire, because it is like fire impossible to satiate. For, it burns down both the visible and the invisible results [of rightious actions].

Sanskrit Shloka Without Transliteration Marks:

avrtam jnanam etena
jnanino nitya-vairina
kama-rupena kaunteya
duspurenanalena ca

Sanskrit to English Word for Word Meanings:

āvṛtam — covered; jñānam — pure consciousness; etena — by this; jñāninaḥ — of the knower; nitya-vairiṇā — by the eternal enemy; kāma-rūpeṇa — in the form of lust; kaunteya — O son of Kuntī; duṣpūreṇa — never to be satisfied; analena — by the fire; ca — also.