A word given to sentinels, and to such as have occasion to visit the guards, used as a signal by which a friend is known from an enemy, or a person who has a right to pass the watch from one who has not; a countersign; a password
Someone's watchword is a word or phrase that sums up their attitude or approach to a particular subject or to things in general. Caution has been one of Mr Allan's watchwords. a word or phrase that expresses an attitude or belief