Jesus nut

Jesus nut is a slang term for the main rotor retaining nut or mast nut, which holds the main rotor to the mast of some helicopters. The related slang term Jesus pin refers to the lock pin used to secure the retaining nut. More generally, "Jesus nut" (or "Jesus pin") has been used to refer to any component that is a single point of failure which results in catastrophic consequences. 
Origin.
The term "Jesus nut" may have been coined by American soldiers in Vietnam; the Vietnam War was the first war to feature large numbers of soldiers riding in helicopters. The term may also have originated with the PBY Catalina, which had two "Jesus bolts" holding the wing into the fuselage.
If the Jesus nut were to fail in flight, the helicopter would detach from the rotor and the only thing left for the crew to do would be to "pray to Jesus." Real examples of the Jesus nut/pin failing are few and far between. However, the nut/pin must be checked before the flight; in 2000, a mast nut was removed to be repainted and was not restored or checked prior to a test flight. The helicopter crashed within ten minutes of takeoff, killing two. Some more recent helicopter systems do not have a Jesus nut.
Other contexts.
More recently, the term has been more generically applied throughout engineering to include any single component whose failure would cause catastrophic failure of the entire system.
Another use for the term is found in rock climbing, in which it refers to the first piece of protection (some of which are also called "nuts") placed on a pitch. This piece must be placed to resist an outward pull as well as a downward pull in order to avoid the possibility of a "zipper", in which the outward pull on the rope from the belayer arresting a falling climber pulls protection pieces from the bottom up. In addition, the Jesus nut prevents the possibility of a factor-two fall onto the belay anchor.
In literature, the term "Jesus nut" was popularized in "Chickenhawk" by Robert Mason, a narrative about his experiences as a pilot in the Vietnam War.