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.

amplify back be tough beef up brace brace up buttress case harden charge clot coagulate confirm congeal crystallize electrify endure energize fortify galvanize generate gird hang tough harden invigorate jell loop in nerve plug in prop refresh reinforce reinvigorate restrengthen rigidify screw up set shock shore up short short-circuit solidify steel step down step up strengthen support sustain switch off switch on tauten temper tense thicken tighten toughen trice up turn off turn on undergird