What is the X in X-mas?
late 14c., from Old French cocatriz, altered (by influence of coq) from Late Latin *calcatrix, from Latin calcare "to tread" (from calx (1) "heel"), as translation of Greek ikhneumon, literally "tracker, tracer."
In classical writings, an Egyptian animal of some sort, the mortal enemy of the crocodile, which it tracks down and kills. This vague sense became hopelessly confused in the Christian West, and in England the word ended up applied to the equivalent of the basilisk. A serpent hatched from a cock's egg, it was fabled to kill by its glance and could be slain only by tricking it into seeing its own reflection. Belief in them persisted even among the educated because the word was used in the KJV several times to translate a Hebrew word for "serpent." In heraldry, a beast half cock, half serpent.
the mediaeval name (a corruption of "crocodile") of a fabulous serpent supposed to be produced from a cock's egg. It is generally supposed to denote the cerastes, or "horned viper," a very poisonous serpent about a foot long. Others think it to be the yellow viper (Daboia xanthina), one of the most dangerous vipers, from its size and its nocturnal habits (Isa. 11:8; 14:29; 59:5; Jer. 8:17; in all which the Revised Version renders the Hebrew _tziph'oni_ by "basilisk"). In Prov. 23:32 the Hebrew _tzeph'a_ is rendered both in the Authorized Version and the Revised Version by "adder;" margin of Revised Version "basilisk," and of Authorized Version "cockatrice."
in the legends of Hellenistic and Roman times, a small serpent, possibly the Egyptian cobra, known as a basilikos ("kinglet") and credited with powers of destroying all animal and vegetable life by its mere look or breath. Only the weasel, which secreted a venom deadly to the cockatrice, was safe from its powers