What it means:
How Shakespeare Used It:
The evil Iago plants doubts in Othello's mind about his wife's faithfulness, while advising him, "O, beware, my lord, of jealousy! / It is the green-eyed monster which doth mock / The meat it feeds on." (Othello, Act 3, Scene 3)
Shakespeare may be evoking cats here - those often green-eyed creatures who toy with their prey before killing it (i.e., mock the meat they feed on).
At the same time, green also suggests not only immaturity, as in a plant's new growth, but also sickly complexions.
".... if jealousy wasn't a factor, three out of every four married people were highly satisfied with the emotional facets of their marriage. However, when the 'green-eyed monster' entered the mix, levels of satisfaction dropped to less than half for married folk." -Michelle Lodge, HealthDay.com, Feb. 12, 2010