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