1. (

• stiffen

) become stiff or stiffer; "He stiffened when he saw his boss enter the room" ;

2. (

• stiffen ,

) 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" ;