What is Action Of A Writ?

Legal Definition
This phrase is used when one pleads some matter by which he shows that the plaintiff had no cause to have the writ which he brought, and yet he may have a writ or action for the same matter. Such a plea is called: a plea to the action of the writ, whereas if it should appear by the plea that the plaintiff has no cause to have action for the thing demanded, then it is called a plea to the action. Termes de la ley.
-- Bouviers Law Dictionary
Legal Definition
A phrase used when a defendant pleads some matter by which he shows that the plaintiff had no cause to have the writ sued upon, although it may be that he is entitled to another writ or action for the same matter. Cowell.
-- Black's Law Dictionary