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stiffen 
verb
- become stiff or stiffer; "He stiffened when he saw his boss enter the room"
Antonyms: loosen, relax, loose - make stiff or stiffer; "Stiffen the cream by adding gelatine"
Antonyms: loosen, loose - restrict; "Tighten the rules"; "stiffen the regulations"
Synonyms: constrain, tighten, tighten up
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