"stiffen" in the verb sense
become stiff or stiffer
"He stiffened when he saw his boss enter the room"
make stiff or stiffer
"Stiffen the cream by adding gelatine"
3. stiffen, tighten, tighten up, constrain
severely restrict in scope or extent
"tighten the rules"
"stiffen the regulations"
Source: WordNet® (An amazing lexical database of English)
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