Become Rigid is a term that describes the process of an object or material becoming more solid and less flexible. OSSIFY (6 letters) is a term that means to become hard and brittle like bone, STIFFEN (7 letters) means to become rigid or firm, HARDEN (6 letters) means to become hard or solid, and SET (3 letters) means to become fixed or established. All of these terms describe the process of an object or material becoming more rigid and less flexible.