Metheglin is ‘a spiced or medicated kind of mead’, apparently a Welsh specialty.  John Drury mentions a letter from George Herbert (1593-1633) to Anne Clifford, Countess of Pembroke, to thank her for a gift of it.

See also John Donne’s comment on the Countess.

Source: John Drury, Music at Midnight: The Life and Poetry of George Herbert (London: Penguin Books, 2014), p. 219