What does the following quote mean? "But when I tell him he hates flatterers; ... He says he does, being then most flattered."

Decius Brutus says this about Julius Caesar in act 2, scene 1 of the play. He is responding to Cassius's comment that it is not certain that Caesar will actually come to the capitol on this particular day because of his superstitions and the fear that something bad will happen to him. Decius Brutus is explaining here that, even if this is what Caesar has decided, the others need not be afraid—Decius Brutus is sure that he will be able to "o'ersway" Caesar, or change his mind. He explains that he will be able to use flattery to do this. The joke in the lines you have quoted is that Decius Brutus is really saying: when I say to Caesar, "Oh, Caesar, I know you hate flatterers!" this comment in itself is an expression of flattery, and is not true. Caesar loves to be told this precisely because he is a lover of flattery, and easily flattered. Decius Brutus may tell him that he is above flattery, but this is the worst kind of flattery there is because it is so completely dishonest.