O, beware, my lord, of jealousy;
It is the green-eyed monster which doth mock
The meat it feeds on.
— Iago, Othello · William Shakespeare
Watch out for jealousy, my lord — it's a green-eyed monster that toys with the very person it consumes.
In one of literature's darkest ironies, Iago warns Othello against the very jealousy he is deliberately planting. The image is exact: jealousy feeds on the person who feels it, mocking them even as it destroys them.