To stiffen is to become stiff or unbending. If your legs stiffen during a long plane flight, you'll want to stretch them out once you land.

You can stiffen something deliberately, like when you stiffen a package with cardboard to protect it in the mail. You can also stiffen in reaction to something: "The spooky creaking sounds upstairs made us stiffen in terror." The earliest sense of this verb was "make steadfast," from a now-obsolete meaning of stiff, "gain strength."

Definitions of stiffen

v make stiff or stiffer

Stiffen the cream by adding gelatine”
loose, loosen
make loose or looser
stiffen with starch
stiffen with or as with buckram
ossify, petrify, rigidify
make rigid and set into a conventional pattern
Type of:
alter, change, modify
cause to change; make different; cause a transformation

v become stiff or stiffer

“He stiffened when he saw his boss enter the room”
loose, loosen, relax
become loose or looser or less tight
become rigid
Type of:
undergo a change; become different in essence; losing one's or its original nature

v restrict

stiffen the regulations”
constrain, tighten, tighten up
Type of:
bound, confine, limit, restrain, restrict, throttle, trammel
place limits on (extent or access)

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