Green’s Dictionary of Slang

a silent breaking of wind.

1593		Nashe Four Letters Confuted in Works II (1883–4) 204: I am afraide your Doctors fart will fall out to be a fatall foyst to your breeches.
1605		Chapman & Jonson Eastward Ho! IV ii: Marry, fyst o your Ruidess. I thought as much.
a.1661		‘The Re-resurrection of the Rump’ in Rump Poems and Songs (1662) II 3: Why a Fart hath a tongue, and a Fyest hath none.
c.1698		B.E. Dict. Canting Crew n.p.: Foyst . . .a close strong Stink, without Noise or Report.
1725		New Canting Dict. [as cit. c.1698].