A snib is a manually operated catch for the internal locking of Yale type locks. Once operated it prevents key holders from using the lock externally, or allows the door to be closed without the lock engaging. Colloquial (UK) also the button inside car doors for locking the car.
The term may also be used (as a noun) to indicate the bolt or latch on a door or window, or (as a verb) to refer to the act of bolting or locking something.
Rosamunde Pilcher uses the term to mean locking windows in her novel, Coming Home. "In the bedrooms, on the landing, windows stood open. She raced from room to room, closing and snibbing every one."
In an episode of The Likely Lads, Terry walks into Bob's house uninvited — Bob: "How did you get in?" Terry: "The door's on the snib"
Says You! used snib in this context on its episode #1415 airing around February, 2012.