And as he fill'd the reeking Vase,The Oxford English Dictionary defines rouser, sense 3, as "A loud noise; a noisy person, song, etc." and cites Swift. But, in plain English, a rouzer here is a fart.
Let fly a Rouzer in her Face.
Kenneth Haynes, English Literature and Ancient Languages (Oxford: Oxford University Press, 2003), p. 58 (with note on p. 182), comments:
The force of those lines depends not only on the scatology but also on the surprising word 'Rouzer'; Geoffrey Hill has observed, 'It would be difficult to find a word that blends the outrageous and the festive more effectively than this.'45
45Geoffrey Hill, The Lords of Limit (Oxford: Oxford University Press, 1984), 78.