This polyhedron was the 26th detailed in Johnson's seminal paper, consequently it's often notated J26, the 26th . It may be formed by joining together two triangular prisms at square sides, ensuring that the axis of one prism is rotated (the "gyro" in the name) with respect to the other. So... the "gyro" in the name is accounted for, the "bi" I assume refers to the two prisms, so what is "fastigium"? Aha!! A bit of googling reveals that fastigium = roof; peak; gable. Cool -- it's a pair of rotated roofs -- got it!