[basilisk]. Her. Two-legged monster or dragon with barbed tongue and cock's head; a reptile/serpent with fatal halitosis and equally fatal stare, from which basilisk-stare is derived. Its feet were those of a cockerel (whereas a *wyvern's were those of an eagle). It was thought to come from a cockerel's egg, being hatched by a serpent. While its breath could kill, it could itself be killed by a weasel. Later, it was a term for 'whore'.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.