I believe it also applies to certain types of rifles that can have the action cycled with the safety on. The idea being to “lock” the action and then “load” as a way to prevent accidental discharges.
That sounds insane.
Why would you cock a rifle and then pour powder down the bore and then begin ramming in the ball, which could cause the action to trigger?
I think the original meaning was lost somewhere and people just apply it to the genre or weapon they’re familiar with.
With the M-16, I was told to “lock” (a magazine in the well) and “load” a round into the chamber.
As a frequent black powder shooter, and shooter of period pieces and flintlocks, I will state you would never do that if you were loading properly, not in 2010, 1810 or in 1710. The piece has a "half cock" position for that.
Maybe you would lock into half-cock, then load.