(1):

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

(2):

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

(3):

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

(4):

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