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Definitions

stiffen

[stif-uhn] / ˈstɪf ən /
VERB
make or become harder
Synonyms
STRONGEST
reinforce solidify stabilize strengthen
STRONG
anneal benumb brace cake candy cement chill clot coagulate condense congeal crystallize curdle firm fix freeze gel harden inflate inspissate jell jelly ossify petrify precipitate prop set starch steady tauten tense thicken
Antonyms
STRONGEST
weaken
STRONG
dilute dissolve heat liquefy melt separate soften thin unfix
WEAK
loosen

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

She has also vowed to stiffen prison sentences and to impose a state of emergency in areas worst hit by crime.

From Barron's • Feb. 1, 2026

In people with Alzheimer's, Aβ accumulates and causes arteries to stiffen, slowing the flow of fluids between brain tissue and cerebrospinal fluid.

From Science Daily • Nov. 11, 2025

Nonetheless, this is an important move that can help stiffen the spines at other schools.

From Salon • Apr. 16, 2025

Now, after shocking France at Twickenham a fortnight ago, they have found a happy habit of finding a way to win as the match teeters, nerves strain and muscles stiffen.

From BBC • Feb. 22, 2025

Bigger, without moving an eyelid, felt his skin grow hot and his muscles stiffen.

From "Native Son" by Richard Wright

From Roget's 21st Century Thesaurus, Third Edition Copyright © 2013 by the Philip Lief Group.