Green’s Dictionary of Slang

1. a gun; thus double-tongued squib, a double-barrelled gun .

c.1838

G.W.M. Reynolds ‘The House Breaker’s Song’ in Farmer Musa Pedestris (1896) 122: A double-tongued squib to keep in awe The chaps that flout at me!

2. a paintbrush .

1845		G.W.M. Reynolds Mysteries of London vol. 2 142: Squibs Paint brushes.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1890–1904		Farmer & Henley Sl. and Its Analogues.

3. a form of sweet made from treacle .

1861

(con. 1840s–50s) H. Mayhew London Labour and London Poor I 203/2: ‘Hardbake’, [...] ‘black balls’, the cheaper ‘bulls eyes’, and ‘squibs’ are all made of treacle.