Green’s Dictionary of Slang

cracker n.12

In phrases

off one’s cracker

1. (US) terrified.

[US]T. Robinson Hard Bounce [ebook] [T]he guy was scared off his cracker.

2. very drunk.

[UK]Guardian 27 Oct. 🌐 ‘We’d have eight hours in the fuckin’ bar’ [...] ‘By the time we went down to the set, we were off our crackers’.