What is Ab Irato?

Legal Definition
Civil law. A Latin phrase, which signifies by a man in anger. It is applied to bequests or gifts, which a man makes adverse to the interest of his heir, in consequence of anger or hatred against him. Thus a devise made under these circumstances is called a testament ab irato. And the suit which the heirs institute to annul this will is called an action ab irato. Merlin, Repert. mots Ab irato.
-- Bouviers Law Dictionary