decus, n. — Green’s Dictionary of Slang

decus n.

[from the Lat. motto decus et tutamen, ‘an ornament and a safeguard’, from Virgil, Aeneid, Bk V, and orig. describing a breast-plate. It was subseq. engraved on coins (where it referred both to the inscription and to its helping to prevent their being clipped) and has reappeared on the English version of the modern £1 coin]

a crown piece, five shillings (25p).

1688		T. Shadwell Squire of Alsatia II ii: I make bold to equip you with some megs, smelts, decus’s and Georges.
c.1698		B.E. Dict. Canting Crew n.p.: Decus c. a Crown or five shilling-piece. The Cull tipt me a score of Decuses, c. my Camerade lent me five Pounds.
1689		‘A Cruel & Bloody declaration’ in A. Carpenter Verse in Eng in 18C Ireland (1998) 40: O tu dulce decus! More sugar sweet than a Dear Joy!
1719		A. Smith Lives of Most Notorious Highway-men, etc. (1926) 205: Decus, a crown, or five shilling piece. The cull tipped me a score of decusses, i.e., my comrade lent me five pounds.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
c.1750–1882		B.M. Carew Life and Adventures.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: He offered me a decus and I nailed him, he offered me a crown and I struck or fixed him.
1811		Lex. Balatronicum.
1822		(con. early 17C) Sir W. Scott Fortunes of Nigel II 282: That noble Master Grahame, whom you call Green, has got the decuses and the smelts.
c.1850		Duncombe New and Improved Flash Dict.