Noah Webster's New International Dictionary of the English Language

1. (v. t.) To make stiff; to make less pliant or flexible; as, to stiffen cloth with starch.

2. (v. t.) To inspissate; to make more thick or viscous; as, to stiffen paste.

3. (v. t.) To make torpid; to benumb.

4. (v. i.) To become stiff or stiffer, in any sense of the adjective.