lock and load

Etymology
Attested since at least 1793, when a document describes flintlock weapons that are ready to fire as "well locked and loaded". The variant "load(ed) and lock(ed)" is found since at least 1815. The phrase may have originated from the use of gunlocks on naval artillery (in use by the Royal Navy since 1745); as gunlocks were not required for firing (a lintstock could be used) it may have been necessary to specify cannon was "locked" as well as loaded.

As an imperative, used since at least 1940, in the U.S. Army Field Manual for the M1 Rifle. Compare e.g. German "laden und sichern" ("load and secure"). Popularized in culture after being used by in  (1949).

Interjection

 * 1) * 1949 — John Wayne in the film Sands of Iwo Jima
 * Lock and load, boy, lock and load.
 * 1)  Prepare for an imminent event.
 * 1)  Prepare for an imminent event.

Verb

 * 1)  To prepare one's weapon.
 * 2)  To prepare for an imminent event.