How we cite our quotes:
Reputation, reputation, reputation! O, I have lost
my reputation! I have lost the immortal part of
myself, and what remains is bestial. My reputation,
Iago, my reputation! (2.3.24)
After Cassio gets into a drunken brawl and loses his position as Othello's officer, he worries about the loss of his "reputation," which is tied up in his military service and his public behavior. Cassio feels that, without his "reputation" as an upstanding soldier, he's nothing more than a "beast."
for whiles this honest fool
Plies Desdemona to repair his fortunes
And she for him pleads strongly to the Moor,
I'll pour this pestilence into his ear,
That she repeals him for her body's lust;
And by how much she strives to do him good,
She shall undo her credit with the Moor.
So will I turn her virtue into pitch,
And out of her own goodness make the net
That shall enmesh them all. (2.3.31)
Earlier, we saw how Cassio sees his "reputation" as being the sum of his public behavior and his military service. Here, we can see that Desdemona's "reputation" hinges on her fidelity to her husband. When Iago says he's going to ruin Desdemona's "credit with the Moor," he means he's going to tarnish her reputation as a loyal wife ("turn her virtue into pitch").
By heaven, I'll know thy thoughts.
You cannot, if my heart were in your hand;
Nor shall not, whilst 'tis in my custody. (3.3.28)
Iago emphasizes that his real thoughts and feelings cannot be known by anyone—not Othello and not even the audience.