How we cite our quotes:
What dost thou say, Iago?
Did Michael Cassio, when you woo'd my lady,
Know of your love?
He did, from first to last: why dost thou ask?
But for a satisfaction of my thought;
No further harm.
Why of thy thought, Iago?
I did not think he had been acquainted with her.
O, yes; and went between us very oft.
Indeed! ay, indeed: discern'st thou aught in that?
Is he not honest? (3.2.14)
This is where Iago plants the seeds of doubt in Othello's mind. Iago suggests that Cassio, who often acted as a go-between when Othello was wooing Desdemona, "went between" Othello and his girl in more ways than one, wink, wink. Iago doesn't come right and say that Cassio and Desdemona have been sneaking around – he implies that something's up and Othello takes the bait.
Why do you speak so faintly?
Are you not well?
I have a pain upon my forehead here. (3.3.2)
After Iago plants the seeds of jealousy in Othello's mind, Othello complains of having a headache, which is a big, big clue that Othello thinks Desdemona's been unfaithful. In sixteenth century literature (Shakespeare's especially), any time a man has a headache or there's some kind of reference to a man having horns growing out of his head, we can be pretty certain there's a reference being made to cuckoldry. A "cuckold" is a man who has been cheated on by his wife, and "cuckolds" are frequently portrayed as having horns. This is why Othello says that married men are "fated" to suffer the "forked plague" (3.3.42) just a few lines earlier.
My name, that was as fresh
As Dian's visage, is now begrimed and black
As mine own face. (3.3.54)
We talk about this passage in more detail in "Race," but it's worth mentioning in our discussion here as well. When Othello suspects that Desdemona is cheating on him with Cassio, he suggests that his "name," or his reputation, is now soiled and "begrimed" because of his wife's supposed infidelity. This idea, that a wife's fidelity to her husband can make or break a man's good reputation is pretty common in Shakespeare's plays. See, for example, The Comedy of Errors, where the fidelity of Antipholus of Ephesus's wife plays such an important role in her husband's good name around town.