Where did the saying lock and load come from?

lock you have to lock the bolt to the rear in order to chamber a round id imagine.

could also mean the quick loading of a general purpose machine gun(gpmg) for firing.

you pull the bolt to the back which then locks into the trigger mechanism, open the top cover to load the ammunition belt. then close top cover. this means the gun is ready of immediate firing. when the trigger is pulled the bolt moves forward picking up the round, moving it to the chamber and firing.