Green’s Dictionary of Slang

speak n.¹

(UK Und.) a stolen item, a robbery.

a.1790		H.T. Potter New Dict. Cant (1795) n.p.: rum speaker or [rum] grab a good booty.
1834		(con. 1737–9) W.H. Ainsworth Rookwood (1857) 114: We’ll overhaul the swag here, when the speak is spoken over.

In phrases

make a speak (v.)

(UK Und.) to commit a robbery.

1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 267: speak committing any robbery, is called making a speak; and if it has been productive, you are said to have made a rum speak.
1827		Egan Anecdotes of the Turf, the Chase etc. 181: Whenever he made a ‘good speak’* he was liberal in the extreme to Sporting betsey. *Robbery.
1842		Egan ‘The By-Blow of the Jug’ in Farmer Musa Pedestris (1896) 144: Very soon she larn’d Jack to make a speak / And he toddled out on the morning sneak.