Webster's Revised Unabridged Dictionary · 1913

Beware

Beware ({not transcribed}) , intransitive verb

[Be, imperative of verb to be + ware. See Ware, Wary.]

1.
To be on one's guard; to be cautious; to take care; -- commonly followed by of or lest before the thing that is to be avoided.
Beware of all, but most beware of man! — Pope
Beware the awful avalanche. — Longfellow
2.
To have a special regard; to heed. [Obsolete]
Behold, I send an Angel before thee.... Beware of him, and obey his voice. — Ex. xxiii. 20, 21

This word is a compound from be and the Old English ware, now wary, which is an adjective. “Be ye war of false prophetis.” Wyclif, Matt. vii. 15. It is used commonly in the imperative and infinitive modes, and with such auxiliaries (shall, should, must, etc.) as go with the infinitive.

Beware (be*wâr") , transitive verb

To avoid; to take care of; to have a care for. [Obsolete]
Priest, beware your beard. — Shakespeare
To wish them beware the son. — Milton