(3 syl.). A monster with the wings of a fowl, tail of a dragon and head of a cock. So called because it was said to be produced from a cock's egg hatched by a serpent. According to legend, the very look of this monster would cause instant death. In consequence of the crest with which the head is crowned, the creature is called a basilisk, from the Greek, basiliskos (a little king). Isaiah says, “The weaned child shall put his hand on the cockatrice den” (xi. 8), to signify that the most noxious animal should not hurt the most feeble of God's creatures. Figuratively, it means an insidious, treacherous person bent on mischief.
“They will kill one another by the look, like cockatrices.” —Shakespeare: Twelfth Night, iii. 4.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894