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"Naked in bed, Iago and not mean harm? it is hypocrisy against the devil" - Othello
Othello says that just being naked together is tempting the devil that they were going to commit adultery but backing off. He suggests that it is not hypocritical for Cassio and Desdemona to ever say anything against the devil as they too have done evil.
"Her honour is an essence that's not seen: They have it very oft that have it not" - Iago
Iago is saying that you cannot see a reputation but many that have them do not deserve them similar to what he said to Cassio in 2.3 "oft got without merit and lost without deserving"
"With her. On her. What you will" - Iago
Iago is purposely provoking Othello, letting his imagination run away with him, saying that because Cassio has been 'lying' next to Desdemona, he may as well have slept with her, its all the same.
"Work on, my medicine work!" - Iago
Iago wills his words to keep working in breaking Othello.
"A horned mans a monster and a beast" - Othello
He is saying a cuckolded man is not a man, he is an animal, this time it is not Iago applying the animalistic language, he does it himself as if trained to do so.
"Marry patience! Or I shall say your all in all spleen and nothing of a man" - Iago
Be patient or you are not a man, merely a wimp. Iago has already stripped him of all that made him manly so that at this point he really doesn't have much to lose.
"Ply Desdemona well and you are sure on't" - Iago to Cassio
Iago is telling Cassio to keep after Desdemona and then he will get his job back, this is quite the opposite of what will happen but works well for Iago in incriminating Desdemona and Cassio.
"I am very villain else" - Iago to Cassio
Iago tells Cassio that Bianca is expecting him to marry her, Cassio doesn't believe him but he said he would admit to be a villain if he was lying.
"I will chop her into messes" - Othello
This is very similar to "I'll tear her all to pieces" 3.3, shows how Othello is very quick to violence, coming the animalistic embodiment of his race's stereotypes.
SD - "He strikes her" - Othello
Throughout the last 2 scenes, Othello has threatened physical violence but it has never come about until now, shows how Iago's taunting has finally got him to breaking point, he is on edge constantly.
"My lord, this would not be believed in Venice" - Lodovico
"Are his wits safe? Is he not light of brain?" - Lodovico
Knowing how highly Othello is regarded in Venice it is not a surprise that those who called him 'valiant' would not believe this, is it also Lodovico saying this wouldn't have occurred in Venice?
Lodovico also questions his sanity and whether he has lost his mind as he cannot see any of the old Othello in this monster.
"I will not stay to offend you" - Desdemona
Desdemona still cares too much for Othello, she will leave so that he can calm down and be happy, she isn't sure of what she has done but she is willing to leave anyway, this shows her kind spirited nature.
"If that the earth could team with women's tears, each drop she falls would prove a crocodile" Othello
Othello doesn't think that Desdemona's emotions are real, she thinks she is crying for sympathy as he doesn't realise she has done nothing wrong and is genuinely hurt by her words.
"It is not honesty to me to speak of what I have seen and known" - Iago
He says that it isn't honesty to just explain what he has seen, also alludes to how he is not actually being honest as he hasn't seen or known anything.
"Goats and monkeys!" -Othello
This is Othello again picking up on Iago's use of animalistic language, this is a shortened version of a quote from Iago in 3.3 "Were they as prime as goats, as hot as monkeys".