Green’s Dictionary of Slang

(UK Und.) a sheet.

c.1698		B.E. Dict. Canting Crew n.p.: Slat c. a sheet.
1725		New Canting Dict. [as cit. c.1698].
1725		Vain Dreamer in Farmer Musa Pedestris (1896) 47: I turned down the lilly slat.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].