Green’s Dictionary of Slang

1. (UK Und.) half-a-crown, 2s 6d (12 ½p), but note cit. 1753: a crown or 5 shillings (25p).

1663		Wandring-Whores Complaint 3: Wand. Who. No rogue in the world could have Pimpt better, give him a Slat for his industry. Pimp. This half-Crown will make me more dilgent.
1703		Hell Upon Earth 6: Slat, Half a Crown.
1714		A. Smith Lives of Most Noted Highway-men, etc. I 209: He taught his Pupil a deal of canting Words, telling him [...] Slat, a Half-Crown.
1729		Life of Thomas Neaves 31: Those Buttocking Frows, that for a Lie buxum, a Hog, or half a Slat, this is six-pence, a Shilling, or half a Crown, shall turn up their Scut to every Porter, Link-boy, Tinker, or Carman.
1741		Canting Academy, or the Pedlar’s-French Dict. 113: Half a Crown A Slat.
1753		J. Poulter Discoveries (1774) 42: I turn and pinch Slats and half Slats; ask Change for Crowns and Half Crowns.
c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions .
1796		Grose Classical Dict. of the Vulgar Tongue (3rd edn).
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. (UK Und.) a guinea, 21 shillings (£1.05p); half a slat, 10/6d (52.5p).

1747

Life and Character of Moll King 12: She flash’d half a Slat, a Bull’s-Eye, and some other rum Slangs. [Ibid.] 24: Half a Slat, 10s. 6d.

3. (US Und.) half a dollar.

1859		Matsell Vocabulum.
1882		Dly Dispatch (Richmond, VA) 1 Nov. 3/3: A ‘slat’ is half a dollar.

4. (US black) $1.

1969		‘Iceberg Slim’ Pimp 87: Here’s a ‘slat’. Get a taste on ‘Dress ’em up Red’.
1997		D.H. Edwards The World Don’t Owe Me Nothing 66: Those guys was only getting a dollar and a half a night. A dollar a night, a slat a night, back in those days!