In the final scene of Shakespeare’s Macbeth, Macbeth, brought to bay by the wronged Macduff, challenges his enemy and nemesis to fight in the last words he speaks in the play:
Lay on, Macduff; And damned be him that first cries, ‘Hold, enough!’
The first three words have given rise to humorous alteration, as in Rider Haggard’s King Solomon’s Mines (1885):
‘Are ye prepared to enter the Place of Death?’ asked Gagool, evidently with a view to making us feel uncomfortable. ‘Lead on, Macduff,’ said Good solemnly, trying to look as though he was not at all alarmed.
From Oxford Dictionary of Quotations.