Lock-and-load Definition

Origin of Lock-and-load

• Originated in American English, supposedly as an instructional command to prepare an M1 Garand, the main rifle used during World War II, for battle. the expression was popularized 1949 by John Wayne in the movie Sands of Iwo Jima. Various similar phrases predate it, including in transposed form as “load and lock”. It is disputed whether the command "lock and load" was ever used by the US military in WWII. The term, "lock and load" was used in the US Army as late as 1969 and was also used in Vietnam.

  From Wiktionary