From Middle English bewar, be war, be ware, forms of Middle English ben ware (“to be on one's guard, be vigilant”, literally “be ware”), equivalent to be + ware. Compare Middle English biwaren, Old English bewarian (“to keep, guard, preserve”).
- (defective, transitive, intransitive) To use caution, pay attention to (used both with and without of).
The verb was traditionally used without of (e.g. "beware the ides of March", from Julius Caesar Act 1, scene 2, 15–19, by Shakespeare), but it is often used with the preposition today.
The verb beware has become a defective verb and now lacks forms such as the third-person singular simple present bewares and the simple past bewared. It can only be used imperatively (Beware of the dog!) or as an infinitive (You must beware of the dog or They told me to beware of the dog).
The inflected forms bewares, bewared, and bewaring are called obsolete in Fowler's Dictionary of Modern English Usage, along with the simple indicative "I beware". The forms bewares and bewared are very rarely found in modern texts, though bewaring is slightly less rare. These inflections are more likely to be found in very old texts.
The meanings of the obsolete inflected forms can be easily understood by replacing "beware" with the more modern equivalent consisting of a conjugated form of "be" and the word "wary". For example "bewares" means the same as "is wary", "bewared" the same as "was wary", etc.