Knuckle
Knuckle turns up in 3 fields: railroad, maritime, shop slang. Here is what it means to the people in each one.
Knuckle in railroad
The hinged, hook-shaped part of a railcar coupler that closes and locks to join two cars.
Two couplers join when their knuckles close around each other and their lock pins drop, and they separate when someone lifts a cut lever to open one. Knuckles are designed as the weak link, so a hard slack run-out breaks a knuckle rather than a car frame. Locomotives carry spare knuckles weighing around 80 pounds, and humping one back to a break, often in the dark, is a rite of passage for conductors.
"We broke a knuckle 60 cars deep, I'm walking back with a new one on my shoulder, give me about forty minutes."
Knuckle in maritime
A sharp, distinct angle or break in the curve of a vessel's hull.
A knuckle is a hard chine or sudden change of angle in the exterior plating of a hull, rather than a smooth, continuous curve. It is commonly found where the topsides meet the flat bottom of a barge or canal boat, or at the stern of certain ship designs. Shipwrights use knuckles to simplify construction with flat steel plates or to deflect spray away from the deck.
"Check the weld integrity along the forward knuckle, we took some heavy pounding there."
Knuckle in shop slang
To smash or scrape the back of the hand when a tool slips.
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.
"I just knuckled myself on the engine block trying to break that alternator bolt loose."