The origin of the term in spaceflight (according to the wiki, at least) is that they were literally constructing full-size / full-weight mock ups of capsules from steel:
"The term boilerplate originated from the use of boilerplate steel[3] for the construction of test articles/mock-ups. Historically, during the development of the Little Joe series of 7 launch vehicles, there was only one actual boilerplate capsule and it was called such since its conical section was made of steel at the Norfolk Naval Shipyard. This capsule was used in a beach abort test, and then subsequently used in the LJ1A flight. However, the term subsequently came to be used for all the prototype capsules (which in their own right were nearly as complicated as the orbital capsules). This usage was technically incorrect, as those other capsules were not made of boilerplate, but the boilerplate term had effectively been genericized.[citation needed]"
The origin of the term in spaceflight (according to the wiki, at least) is that they were literally constructing full-size / full-weight mock ups of capsules from steel:
"The term boilerplate originated from the use of boilerplate steel[3] for the construction of test articles/mock-ups. Historically, during the development of the Little Joe series of 7 launch vehicles, there was only one actual boilerplate capsule and it was called such since its conical section was made of steel at the Norfolk Naval Shipyard. This capsule was used in a beach abort test, and then subsequently used in the LJ1A flight. However, the term subsequently came to be used for all the prototype capsules (which in their own right were nearly as complicated as the orbital capsules). This usage was technically incorrect, as those other capsules were not made of boilerplate, but the boilerplate term had effectively been genericized.[citation needed]"