The Jesus Pin is a common aviation term for the pin that holds the rotor blades to the drive shaft of a helicopter.
The term is a reference to the eventuality of this pin failing, at this point the rotor blades will detach from the chassis, and the helicopter will assume the aerodynamic properties of a brick. Then the only thing left to do is pray to Jesus, as divine intervention is your best hope to avoid a sudden, fiery death.
The slang (also Jesus Nut) was originally used during the Vietnam War, a period where more soldiers than ever before put their faith in the Jesus Pin on a daily basis.
More recently engineers have expanded the term to include any single point (pin, nut, bolt, or clamp) whose demise will result in catastrophic failure of the entire mechanical device, to the detriment of the personnel whose lifes dependent on it.
Also referred to as single point catastrophic failure or criticality-one.