Oh, beware, my lord, of jealousy!
It is the green-eyed monster which doth mock
The meat it feeds on. (III.iii.)

As Iago makes insinuations about Desdemona’s adultery and Othello pressures him to reveal what he knows, Iago warns Othello against succumbing to jealousy. Of course, Iago issues this warning with a false earnestness. That is, he knows that saying the word “jealousy” and conjuring an offensive visual image will intensify Othello’s concern. Iago’s anthropomorphizing of jealousy as a “green-eyed monster” is famous, and his use of the color green stems from a Renaissance belief that green was a “bilious hue,” linked to an imbalance of the humors that caused fear and jealousy.

Trifles light as air
Are to the jealous confirmations strong
As proofs of holy writ. (III.iii.)

Iago delivers these lines in a short soliloquy where he informs the audience of his plan to plant Desdemona’s handkerchief in Cassio’s room. Here, the handkerchief is the “trifle light as air.” In itself the handkerchief signifies nothing. However, knowing that Othello’s jealousy has already ramped up, Iago predicts that he will overestimate the handkerchief’s significance, taking it as a “proof of holy writ”—that is, as evidence direct from the Bible.

But jealous souls will not be answered so.
They are not ever jealous for the cause,
But jealous for they’re jealous. It is a monster
Begot upon itself, born on itself. (III.iv.)

Emilia says these words to Desdemona in an attempt to explain the irrational nature of jealousy. Even though jealous individuals may state specific reasons for their jealousy, jealousy has no cause but itself. In other words, jealous people are inherently jealous. Emilia signals the circularity of this logic with the confounding image of jealousy as a monster that gives birth to itself. Emilia’s image recalls Iago’s “green-eyed monster.” It also recalls the ancient symbol of the ouroboros, which depicts a snake swallowing its own tail and therefore stuck in a self-perpetuating loop.