Green’s Dictionary of Slang

spice n.¹

(UK Und.) mugging, footpad robbery.

c.1790		Jolly Blade [broadside ballad] And when my cash it did run low / Straight to the spice was forced to go.
1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 267: spice the spice is the game of footpad robbery; describing an exploit of this nature; a rogue will say, I spiced a swell of so much, naming the booty obtained. A spice is a footpad robbery.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1812].
1834		(con. 1737–9) W.H. Ainsworth Rookwood (1857) 247: A lucky spice on the road set them up.

In compounds

spice gloak (n.) (also spiceman) [gloak n.]

a footpad, a highwayman, a mugger.

1812		Vaux Vocab. of the Flash Lang.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1825		Pierce Egan’s Life in London 1 May 108/2: [S]he cannot portray the knowing saucy Spiceman, who relies on his pluck and his pops for all his movements in life.
1830		W.T. Moncrieff Heart of London II i: Cracksmen, buzmen, scampsmen, we [...] On the spice gloak high toby / We frisk so rummy, / And ramp so plummy.
1832		Public Ledger 12 Nov. 3/3: The principal actors in criminality may be classed under the following heads: Footpads Spicemen.