loonslate, n. — Green’s Dictionary of Slang

loonslate n.

also loonslatt

[? loon n.¹ + slat n.² (1); lit. ‘a fool’s half-crown’]

one shilling and one penny-halfpenny, 1s 1½d.

c.1698		B.E. Dict. Canting Crew n.p.: Loon-slatt, a Thirteen Pence half Penny.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Loonslate, thirteen pence halfpenny.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1785].