The Oxford Thesaurus An A-Z Dictionary of Synonyms - stiffen v.

1) thicken, coagulate, clot, harden, jell, set, solidify, congeal, crystallize: Beat the egg-whites with a whisk until they stiffen. 2) brace, reinforce, tauten, rigidify, toughen, strengthen: The weak support was stiffened by means of a steel bar.