In many various fields of engineering, a brassboard is one step up from a breadboard. It is a working prototype ready for field testing and arranged broadly in the final form, but is still not fully worked out in all its details. The exact definition is variable from field to field, but it generally indicates that "we have something which works the way it is supposed to, but we're not done tweaking it yet".
Brassboards get their name from the use of precisely machined prototypes often made of brass, as opposed to breadboards, which were originally made quite literally of wood. They are much more inflexible in their layout, as all major design changes have already been made. Brassboards are particularly referred to in electrical and computer engineering, where precise distances and layout can be very important.
Brassboard has an even more specific meaning when it comes to designing integrated circuits; in System on Chip design (think Raspberry Pi), prototypes go through four stages: engineering validation boards (EVB), brassboards, wingboards, and final pre-production samples. These are industry-standardized terms, and correspond to the size of the boards used, with the wingboards being four times as large as the final product, and the brassboards four times as big as the wingboards. The giant EVB is used for silicon validation and testing out configuration options; the brassboard is used primarily for software testing; wingboards test the final components and software that will be used in mass production.