The definition of countersue is when someone is suing you and you turn around and sue them.(verb)
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.
See countersue in Webster's New World College Dictionary
See countersue in American Heritage Dictionary 4
transitive verb coun·ter·sued, coun·ter·su·ing, coun·ter·sues