What is the allusion to a cockatrice in Romeo and Juliet?

The cockatrice was a legendary monster with the body of a serpent and the head, wings and legs of a cock. The allusion to a cockatrice by Juliet occurs in Act III, Scene 2 of Romeo and Juliet. The Nurse's behavior and speech have led Juliet to believe at first that Romeo is dead. Juliet asks:

Hath Romeo slain himself? Say thou but ‘Ay,’
And that bare vowel ‘Ay’ shall poison more
Than the death-darting eye of cockatrice.

In medieval times the mythical cocatrice was supposed to have the power to kill with a glance of its eye or a touch from its loathsome body. The Nurse is so distraught that she is inarticulate. She says more ambiguous words that increase Juliet's fears for Romeo. The Nurse is not only concerned about the death of Tybalt but about the consequences for Romeo and for her Juliet. She tells Juliet that Romeo has been banished because of his violation of the Prince's ultimatum against fighting in the streets. This interchange between Juliet and the Nurse is both sad and comical, because the audience knows exactly what happened but the Nurse cannot find the words to convey the facts and Juliet gets them all garbled in transmission.