from The Century Dictionary and Cyclopedia
- n. A bolt or bar used to fasten a door at night.
- n. A spring-bolt in a lock which can be opened by a knob from inside the door, but only by a key from the outside.
Sorry, no etymologies found.
"No; for the night-bolt is controlled by an electric button, which you have to push before it can be moved from the inside, so no one not familiar with the house could have opened it."
Well, when Pedro opposed their passage and Frank ran out, the two fled, finding the night-bolt off at the street door.