The term itself derives from the components of a musket: the 'lock' being the firing mechanism, 'stock' being the wooden butt-end of the gun, and 'barrel' being the cylindrical component, concurring 'all' the components of the gun.

Another interpretation could be business related: i.e., one would own "the lock" as in the building, stock as in the stock exchange, investments, etc., and "barrel" as in all inventory, etc.

The term was first recorded in the letters of Sir Walter Scott in 1817, in the line "Like the High-landman's gun, she wants stock, lock, and barrel, to put her into repair". It is, however, thought that this term evolved into a popular saying some years before in England.