Drury tells us that ‘according to Pliny and alchemical tradition, this was a mythical serpent, coiled and fiery, perversely hatched from a cock’s egg’. George Herbert (1593-1633) uses it as a metaphor in his poem ‘Sin’s Round’:
‘Sorry I am, my God, sorry I am,
That my offences course it in a ring.
My thoughts are working like a busy flame,
Until their cockatrice they hatch and bring… ‘
The OED describes it as ‘A serpent, … fabulously said to kill by its mere glance, and to be hatched from a cock’s egg’. In heraldry, it is ‘a hybrid monster with head, wings, and feet of a cock, terminating in a serpent with a barbed tail’. An obsolete sense is as an insult to a woman, such as ‘prostitute’ or ‘whore’.
For a visual definition, see this dedicated Pinterest board.
Source: John Drury, Music at Midnight: The Life and Poetry of George Herbert (London: Penguin Books, 2014), p. 76