Sounds military to me, but I can’t think of anything that you can load, after having locked first.
Anybody?
From this source:
http://www.wordorigins.org/wordorl.htm
Lock and Load
This imperative phrase originally referred to the operation of the M1 Garand Rifle, the standard U.S. Army rifle of WWII. Its meaning is more general now, referring to preparation for any imminent event.
The original phrase was actually reversed, “load and lock.” The phrase refers to inserting a clip of ammunition into the rifle, “loading,” and “locking” the bolt forward thereby forcing a round into the chamber. The phrase first appears in Gach’s 1941-42 In the Army Now. It was immortalized by John Wayne (who else?) in 1949’s Sands of Iwo Jima, where the Duke reversed the phrase to the current “lock and load.”
The term “lock” in this phrase is a different use of the word than in references to the firing mechanism of a weapon, as in “flintlock.”
It’s supposed to be “load and lock.”
One version of the origin was that John Wayne accidentally reversed the words in one of his combat movies (Sands of Iwo Jima?) and that it made it into the popular consciousness, overwhelming the correct order. (Not sure that I can confirm the story, but I’ll go look.)
Here is the Word Maven on Lock and load (and I see that KidCharlemagne beat me with a cite.