Bhagavad Gita 3.37 is Chapter 3, Verse 37, in Karma Yoga (Path of Action). Below are the Sanskrit text, a transliteration, a plain-English translation and an explanation.
śrī-bhagavān uvāca kāma eṣa krodha eṣa rajo-guṇa-samudbhavaḥ mahāśano mahā-pāpmā viddhy enam iha vairiṇam
"The Supreme Lord said: It is desire, it is anger, born of the quality of rajas. Know this to be the all-devouring, sinful enemy."
What This Means:
Krishna names the enemy: Desire (kama) and its companion, anger (krodha). They come from rajas, the quality of passion and agitation. This enemy devours everything, causes great harm, and is your true adversary.
Going Deeper:
Desire and anger are presented as two forms of one force. Desire (kama) is the wanting of what we don't have; anger (krodha) arises when desire is frustrated. Both emerge from rajas-guna (the mode of passion, activity, ambition). 'Mahaashana' (great eater) shows its insatiable nature, desire is never satisfied. 'Mahapapma' (great sinner) shows it leads to endless wrongdoing. 'Vairinam' (enemy) makes it clear: this is what you're fighting.
How To Apply This:
Your real enemy isn't outside, it's desire and its shadow, anger. Every frustration, every conflict, every regret usually traces back to wanting something you didn't get. Notice desire arising. Notice how anger follows when desire is blocked. This is your opponent. Know your enemy.
Where this verse sits
In 3.36 Arjuna asks a question everyone has asked: by what is a person driven to do wrong, even against their own will, as if pushed? Verse 37 is the beginning of Krishna's answer, which runs to the end of the chapter. Verses 38 to 40 describe how desire covers knowledge, as smoke covers fire and dust a mirror, and where it lives: in the senses, the mind and the intellect. Verse 41 gives the first instruction, to restrain the senses, and verses 42 and 43 the hierarchy: senses, then mind, then intellect, and above the intellect what Shankara takes to be the self and Ramanuja takes to be desire itself, the obstacle to be known and slain; from that steadiness Arjuna is told to kill the enemy.
What the words say
Kama esha krodha esha, 'this is desire, this is anger', names one enemy with two faces; anger is what desire becomes when it is blocked. Rajo-guna-samudbhavah traces both to rajas, the guna of restlessness and appetite. Mahashana, 'the great eater', says it cannot be filled; Shankara adds that this is why it cannot be satisfied by giving it what it wants. Maha-papma, 'the great sinner', says it is the source of wrong action, which answers Arjuna's question. Viddhi enam iha vairinam, 'know this to be the enemy here', turns a feeling into an opponent, something to be watched and fought rather than simply felt.
A common misreading
The verse does not say that wanting anything is sin. The Gita elsewhere praises desire that is in accord with dharma, and in 7.11 Krishna calls himself that desire. What is condemned is kama as an appetite that consumes and does not stop, and the anger that follows it. The verse should be read with 2.62 and 2.63, which trace the whole descent: dwelling on objects, attachment, desire, anger, confusion, loss of memory, ruin. The enemy is identified so that the chain can be broken at its first link, at the dwelling, before desire has grown.
Key Sanskrit Terms: