Bg 3.39

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

Word for word: 
avrtam — covered; jnanam — pure consciousness; etena — by this; jnaninah — of the knower; nitya-vairina — by the eternal enemy; kama-rupena — in the form of lust; kaunteya — O son of Kunti; duspurena — never to be satisfied; analena — by the fire; ca — also.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
It is said in the Manu-smrti 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.