Antonyms for the word "most unpierceable" include "easily penetrable," "permeable," "piercable," and "porous." These words suggest that the object or material in question can be pierced or breached with relative ease. They are often used to describe materials such as fabrics, membranes, and skins that can be penetrated by sharp objects, liquids, or gases. On the other hand, the term "most unpierceable" implies that the object is incredibly resistant to any attempts to penetrate or breach it, making it highly durable and robust. Such materials might include metals, alloys, and composites designed for use in high-performance applications.