Stiffen/ˈstɪfn/ Definition 1 of 3Pro Tip 1/3Physical Change Describes literal hardening or solidifying of materials or objects, often over time. Mix the ingredients until they stiffen into dough. Pro Tip 2/3Metaphoric Use Used figuratively to describe firming up things like plans, positions, or attitudes. The politician had to stiffen his stance on climate policy. Pro Tip 3/3Body Language Can indicate a sudden physical response to surprise or discomfort, often involuntary. His jaw stiffened when he heard the unexpected noise.