Green’s Dictionary of Slang

1. a beggar who searches for cigarette ends.

1984

Partridge DSUE (8th edn) 323/1: C.20.

2. (drugs) the residue of poor-quality opium or heroin.

1937		B. Dai Opium Addiction in Chicago 198: Dog. The residue left from gum when making smoking opium.
1937		‘Boxcar Bertha’ Sister of the Road (1975) 116: The residue, known as the ‘dog,’ consisting of leaves, twigs, and stems from the plants [...] is thrown away.
1968		‘Sl. of Watts’ in Current Sl. III:2.