When your neighbor sues you for trespassing and you sue your neighbor for encroaching on your land, this is an example of a time when you countersue.
transitive verbcoun·ter·sued, coun·ter·su·ing, coun·ter·sues Law
(third-person singular simple present countersues, present participle countersuing, simple past and past participle countersued)
- (intransitive) To sue a person or entity who is suing one.
counter- + sue