Etymology

Uncertain.

The most commonly accepted origin traces this order to the operation of the M1 Garand rifle. Before loading the ammunition clip into the rifle, the operating rod handle is pulled to the rear until the bolt is securely locked open. According to the M1 Garand Manual, loading the clip without first locking the bolt could result in an accidental discharge of a round. In the 1943 training film (Rifle Marksmanship with M1 Rifle) the instructor orders first "Lock" then "Load".

Alternate theories include:

• A transposition of "load and lock" - to load the ammunition clip into the rifle, then to lock the bolt forward (which forces a round into the chamber, readying a semi-automatic rifle for firing).
• The use of flintlock rifles, which required the hammer to be locked back at the half-cock position before placing primer in the pan.
• Alternatively, a Sporting Magazine from 1821 had the complete flintlock expression as "brush the dirt away from the lock, and load ...".http://books.google.com/books?id=AL0CAAAAYAAJ&pg=PA126&dq=%22lock+and+load%22+date:1530-1850&as_brr=0
• From artillery usage: to 'lock' a gun into firing position before loading.

Verb

1. A military command to put a weapon's safety catch on, and load it with ammunition.
   • 1949 — John Wayne in the film Sands of Iwo Jima

   Lock and load, boy, lock and load.

2. To prepare for an imminent event.
3. A military command to open the bolt of a machine gun (Lock Open) and load it. This is because most heavy machine guns, as well as the Browning Automatic Rifle, and many submachine guns such as the Thompson, and the M3 "grease gun" fire from an open bolt.