Green’s Dictionary of Slang

1. a heavy blow.

1825		Pierce Egan’s Life in London 10 July 189/1: Cooper [...] put a cracker on the jaws of the former hero .
1833		Satirist (London) 20 Jan. 445/2: And woe to my sconce if my laugh becomes slacker / [...] / My head very oft is exposed to a cracker.
1858		A. Mayhew Paved with Gold 189: The first round was soon terminated, for Jack got a ‘cracker on his nut’ which knocked his ‘rammers’ from under him.
1914		Truth (Melbourne) 24 Jan. 11/7: Though he got the side of the head [...] with the the best kind of right smash he owns, he could not feaze, let alone stop the packet of crackers from Yarra’s banks .
2001		I. Welsh Glue 88: Begbie’s brar elbays a boy a sneaky cracker in the side ay the heid.

2. a fall, lit. or fig.

1869		Leamington Advertiser 11 Nov. 5/4: Poor Tom, sir, did all he knew to get a little bit of paper done, but couldn’t bring it off, and now he's gone a cracker over head and ears, and must bolt or pay up.
1984		Partridge DSUE (8th edn) 264/1: from ca.1865: ob.