“O, beware, my lord, of jealousy;
It is the green-ey’d monster, which doth mock
The meat it feeds on.”
― William Shakespeare, Othello (Act 3, Scene 3)
Iago is warning Othello not to be a jealous man because jealousy (the green-eyed monster) will interfere with his mind (his imagination is what feeds it)
Jealousy is called the “green eyed monster” because green was the colour associated – in Renaissance England – to jealousy and envy. It was connected with bile, one of the bodily fluids or “humours” that made up the temperament. It is the colour of seriously ill people’s skin and of unripe fruits that cause stomach pains. This phrase was possibly coined by Shakespeare to denote jealousy, and used also in The Merchant of Venice, (“green-eyed jealousy”). It may allude to cats, often green-eyed, who play with their prey before killing it.
The word JEALOUSY stems from the French jalousie, which in turn derives from Low Latin zelosus (full of zeal), and the Greek word ζήλος (ardour, zeal), with a root connoting “to boil, ferment”; or “yeast”.
Originally it defined a condition of zealous emulation, and later of resentment at being (or believing that one is or may be) supplanted or preferred in the love or affection of another, or in the enjoyment of some good originally a form of envy, associated with a feeling of personal claim.
However, envy is the emotion when you want someone else’s possession, whereas jealousy is the emotion when you fear you may be replaced in the affection of someone you love or desire.
Othello was jealous of Desdemona, but Iago was envious of Othello