To violently strike one's hand against a hard metal surface, usually because a wrench or pry bar suddenly slips off a stubborn fastener. It results in scraped skin, bruised joints, and immediate swearing. Mechanics and fitters expect to do this regularly when working in tight, rusty spaces.
Heard on the job
I just knuckled myself on the engine block trying to break that alternator bolt loose.
Also heard as
- Bust a knuckle