In "Othello," What Is "The Green-eyed Monster"?

Shakespeare makes at least two references to the 'green eyed monster'. The first is in Othello, when Iago says "O beware my lord of jealousy: it is the green eyed monster which doth mock the meat it feeds on". In the Merchant of Venice jealousy is again mentioned "cats, lions, togers and all the green eyed tribe mock the meat they feed on".
It is thought that Shakespeare was making particular reference to the way a cat will cruelly play wiht a mouse, bird etc before it kills it. It will be tender towards the mouse, carrying it, gently tapping it, offering it the chance to escape only to recapture it again etc. But ultimately it hates the mouse and will kill it.
So when Shakespeare describes the green eyed monster he is talking not just about the love hate relationship that cats have with their prey, but also how a person who is jealous of their lover will have a 'love hate' relationship with that person. Because they love them, they also hate them and so they have become that green eyed monster!

Iago, the General Othello's standard bearer, warns him "Beware, my lord, of jealousy/ It is the green-eyed monster, which doth mock/ The meat it feeds on." Even in Shakespeare's time, green was traditionally associated with jealousy, as it is today: "green with envy." Jealousy is very much the theme of Othello; the plot is driven by Iago's own envy of Cassio, Othello's lieutenant ("He has a daily beauty in his life/ That makes me ugly") and of Othello's happiness with his bride, Desdemona ""I'll set down the pegs that make this music," he mutters to himself when he sees them together.) To feed his own jealousy he instils it in Othello, convincing him that Desdemona and Cassio are having an affair. At first Othello resists, but Iago plays so cleverly on his insecurities about his age, race and ignorance of Venetian culture ("In Venice they let heaven se the pranks/ They dare not show their husbands") that he is gradually driven into a violent rage and kills his wife.