harden
verb
hard·en ˈhär-dᵊn 
hardened; hardening ˈhärd-niŋ
ˈhär-dᵊn-iŋ
transitive verb
1
: to make hard or harder
2
: to confirm in disposition, feelings, or action
especially : to make callous
hardened his heart
3
a
: inure, toughen
harden troops
b
: to inure to unfavorable environmental conditions (such as cold) —often used with off
harden off seedlings before transplanting
4
: to protect from blast, heat, or radiation (as by a thick barrier or placement underground)
intransitive verb
1
: to become hard or harder
2
a
: to become firm, stable, or settled
b
: to assume an appearance of harshness or severity
her face hardened at the thought
3
: to become gradually acclimatized to unfavorable conditions —often used with off
plants hardened off before the first frost
Synonyms
- concrete
- congeal
- firm (up)
- freeze
- indurate
- set
- solidify
See all Synonyms & Antonyms 
Example Sentences
The presence of certain substances in the blood can cause the arteries to harden. These additives are designed to harden the steel. substances that can harden the arteries The news has hardened opposition to the government. He had been hardened by his years of military service.
Recent Examples on the Web Writers seem to be especially vulnerable in polarized times when beliefs harden into dogma and those who hold opposing views are demonized. 
Jonathan Bate, The Conversation, 26 Aug. 2022 Soy candles usually need at least 4 hours to solidify, but the wax in a large vessel like a bucket requires at least 24 hours to settle and harden. Averi Baudler, Better Homes & Gardens, 12 Aug. 2022 This week, Newsom also urged California lawmakers to harden the state’s climate and energy goals. Amy Hubbard, Los Angeles Times, 13 Aug. 2022 Now is the time for CPOs to harden their processes and gird their teams for the next seismic shift. Richard Waugh, Forbes, 12 Aug. 2022 Now, the revelation that the Taliban were sheltering Al Qaeda’s leader is likely to harden support for sanctions. New York Times, 2 Aug. 2022 Chinese Foreign Ministry Spokesperson Zhao's comments on Monday were not the only sign that Beijing is watching the election, the outcome of which has the potential to further harden the British line on China. Simone Mccarthy, CNN, 26 July 2022 Russian forces have now occupied the Kherson region for nearly five months and have been largely unmolested in their efforts to harden military positions and prepare for an assault. Michael Schwirtz, BostonGlobe.com, 25 July 2022 In Indian Territory, what had been a set of highly varied, sometimes kin-adjacent forms of enslavement began to harden, and Indian attitudes and practices edged closer to those of white Americans. Philip Deloria, The New Yorker, 18 July 2022 See More
Jonathan Bate, The Conversation, 26 Aug. 2022 Soy candles usually need at least 4 hours to solidify, but the wax in a large vessel like a bucket requires at least 24 hours to settle and harden. Averi Baudler, Better Homes & Gardens, 12 Aug. 2022 This week, Newsom also urged California lawmakers to harden the state’s climate and energy goals. Amy Hubbard, Los Angeles Times, 13 Aug. 2022 Now is the time for CPOs to harden their processes and gird their teams for the next seismic shift. Richard Waugh, Forbes, 12 Aug. 2022 Now, the revelation that the Taliban were sheltering Al Qaeda’s leader is likely to harden support for sanctions. New York Times, 2 Aug. 2022 Chinese Foreign Ministry Spokesperson Zhao's comments on Monday were not the only sign that Beijing is watching the election, the outcome of which has the potential to further harden the British line on China. Simone Mccarthy, CNN, 26 July 2022 Russian forces have now occupied the Kherson region for nearly five months and have been largely unmolested in their efforts to harden military positions and prepare for an assault. Michael Schwirtz, BostonGlobe.com, 25 July 2022 In Indian Territory, what had been a set of highly varied, sometimes kin-adjacent forms of enslavement began to harden, and Indian attitudes and practices edged closer to those of white Americans. Philip Deloria, The New Yorker, 18 July 2022 See More Word History
First Known Use
13th century, in the meaning defined at transitive sense 1
Phrases Containing harden
Kids Definition
harden
verb
hard·en ˈhär-dᵊn
hardened; hardening
1
: to make or become hard or harder
By now the cement has hardened.
2
: to make or become hardy or strong Years of tough work had hardened his muscles.
3
: to make or become stubborn or unfeeling
She hardened her heart and refused to forgive him.
Biographical Definition
Harden 1 of 2
biographical name (1)
Har·den ˈhär-dᵊn
Sir Arthur 1865–1940 English chemist
Harden
2 of 2biographical name (2)
Maximilian 1861–1927 originally Felix Ernst Witkowski German writer
harden
verb
1
as in to freeze
to become physically firm or solid the glue begins to harden as soon as it is exposed to air Synonyms & Similar Words
Relevance
- freeze
- stiffen
- solidify
- congeal
- indurate
- concrete
- thicken
- set
- gel
- crystallize
- encrust
- jelly
- jell
- coagulate
- gelatinize
- cake
- calcify
- firm (up)
- petrify
- ossify
- rigidify
- callus
- crystalize
- anneal
- clot
- temper
- incrust
- gelate
- case-harden
Antonyms & Near Antonyms
- soften
- liquefy
- melt
- dissolve
- liquify
- thaw
- smelt
- fuse
- flux
- unfreeze
- deliquesce
2
as in to strengthen
to make able to withstand physical hardship, strain, or exposure pioneer women who had been hardened by years of living on the plains Synonyms & Similar Words
- strengthen
- toughen
- fortify
- steel
- indurate
- inure
- adjust
- season
- adapt
- bolster
- acclimate
- anneal
- reinforce
- temper
- invigorate
- acclimatize
- brace
- vitalize
- enforce
- immunize
- accustom
- habituate
- buttress
- condition
- reenforce
- forearm
- break in
- support
- boost
- naturalize
- train
- prop (up)
- limber (up)
Antonyms & Near Antonyms
- soften
- weaken
- exhaust
- sap
- emasculate
- cripple
- incapacitate
- sensitize
- enfeeble
- hamstring
- enervate
- debilitate
3
as in to toughen
to increase the ability of (as a muscle) to exert physical force arm muscles that were hardened by all the years of casting and hauling his fishing nets Synonyms & Similar Words
- strengthen
- toughen
- fortify
- anneal
- temper
- energize
- invigorate
- vitalize
- beef (up)
- restrengthen
- firm (up)
- tone (up)
Antonyms & Near Antonyms
- weaken
- hurt
- impair
- paralyze
- cripple
- injure
- harm
- debilitate
- enervate
- damage
- enfeeble
- undermine
- incapacitate
- undercut
- break down
- sap
- wear out
4
as in to stiffen
to make more harsh, uncompromising, or severe the government hardened restrictions on travel to and from the war-torn nation Synonyms & Similar Words
- stiffen
- strengthen
- toughen
- harshen
Antonyms & Near Antonyms
- ease
- soften
- lessen
- ameliorate
- moderate