To make stiff; to make less pliant or flexible; as, to stiffen cloth with starch.
To inspissate; to make more thick or viscous; as, to stiffen paste.
To make torpid; to benumb.
To become stiff or stiffer, in any sense of the adjective.
become stiff or stiffer; "He stiffened when he saw his boss enter the room"
make stiff or stiffer; "Stiffen the cream by adding gelatine"
restrict; "Tighten the rules"; "stiffen the regulations"