In such a time as this it is not meet
That every offense should bear his comment.
Let me tell you, Cassius, you yourself
Are much condemn'd to have an itching palm,
To sell and mart your offices for gold
Cassius:Julius Caesar Act 4, scene 3, 7–12
I, an itching palm!
"An itching palm" is one that must be scratched with coins. Brutus's dermatological metaphor implies that Cassius's desire for gold—which leads to the sale ("mart") of favors—is unconscious and compulsive. Cassius doesn't think, he just scratches.
Brutus and Cassius his co-commander, are preparing for battle with Marc Antony and Octavius, who are bent on avenging the murder of Julius Caesar and on securing power in Rome. This is no time, as Cassius says, for comment on every petty offense. But Brutus is too angry—he thinks Cassius deliberately withheld necessary finances from him—to let the matter rest. The two will eventually make amends, after elaborate (and stereotypically Roman) gestures toward suicide.