What was lago's reply when Othello demanded proof of his wife's disloyalty?

Answer 1

He responded "How likely is it that I'll be able to show them to you actually in bed together??" As Iago well knew, this would not only be difficult but actually impossible since Desdemona and Cassio were not really having an affair. But as we know from people who believe in conspiracy theories, and Iago knew then, sometimes you can bamboozle people into believing that the fact that something is difficult to prove is in itself proof. Unfortunately it works on Othello.

Answer 2

"Would you, the supervisor, grossly gape on, behold her tupp'd?" In other words, "What kind of proof do you want, Othello? Do you want to watch them having sex?" In this way, Iago prepares Othello to accept much less probative and much more ambiguous evidence.

Answer 3

Iago says, "Would you, the supervisor, grossly gape on, behold her topp'd?", which is to say "Do you want to actually watch Cassio screwing your wife?" This has the double effect of reinforcing the image that Othello is trying to keep out of his mind, of Desdemona and Cassio making love, and at the same time excusing the flimsy proof which Iago offers of Desdemona's infidelity.