After being sued, she decided to countersue for libel.

countersue 是由前缀 counter-（表示"反对、对抗"）和动词 sue（起诉）组成的动词。counter- 表示相反或对应的动作，sue 意为"提起诉讼"。 countersue 指"反诉"，即被告对原告提起的诉讼进行反向起诉的行为。