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

• freeze
• stiffen

Examples of harden in a Sentence

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

The confession struck him as both raw and deeply tragic, a glimpse of how ambition hardens into captivity. —Eva Roytburg, Fortune, 26 Oct. 2025

Fertilization and pruning encourage new growth, which does not have time to harden off before cold temperatures set in. —Kim Toscano, Southern Living, 24 Oct. 2025

The finale opens in the stillness that follows, grief hardening into vengeance. —Jp Mangalindan, Time, 24 Oct. 2025

The ad, purchased by the government of Ontario and broadcast on major US television networks, aired clips of the address, delivered from Camp David, where Reagan was soon to meet the prime minister of Japan at a time when American attitudes toward Japan were hardening. —David Goldman, CNN Money, 24 Oct. 2025

Word History

First Known Use

13th century, in the meaning defined at transitive sense 1

Phrases Containing harden

• case-harden
• harden one's heart

Rhymes for harden

• cardan
• garden
• pardon

Kids Definition

harden

verb

hard·​en ˈhärd-ᵊn 

hardened; hardening ˈhärd-niŋ 

-ᵊn-iŋ

1

: to make or become hard or harder

2

: to make unfeeling or unsympathetic

hardened his heart

3

: to make or become hardy or strong

muscles hardened by exercise

4

: to protect from blast, heat, or radiation (as by a thick barrier or by placing underground)

hardened missile sites

5

a

: to become firm, stable, or settled

b

: to express harshness

hardener

ˈhärd-nər

-ᵊn-ər

noun

Biographical Definition

Harden 1 of 2

biographical name (1)

Har·​den ˈhär-dᵊn 

Sir Arthur 1865–1940 English chemist

Harden

2 of 2

biographical name (2)

Maximilian 1861–1927 originally Felix Ernst Witkowski German writer