A countercharge is what you throw back when someone has already said you did something wrong. They accuse you; you accuse them. It is not a first complaint. It is the second shot, and people often hear it as a way to change the subject or to make both sides look dirty. You meet it in court, in politics, and in the news: one side files a charge, the other answers with a countercharge. It can be a noun (the statement itself) or a verb (to make that statement). It is fairly formal; in ordinary talk people more often say they accused the other person back. Though actually, a countercharge can be true, or it can be a tactic. The word itself does not tell you which.