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

Iran rejects tight restrictions on its nuclear program and has hardened its stance after Israel and the U.S. targeted its nuclear facilities in June. —Amira El-Fekki‎, MSNBC Newsweek, 8 July 2025

They’d be hardened to anything that life and work threw their way. —Jodie Cook, Forbes.com, 7 July 2025

Once plants are hardened off, they can be transplanted into the garden. —Kim Toscano, Southern Living, 3 July 2025

Though fewer than Iran, Israel’s forces have been battle hardened by regional conflicts. —Christopher Bodeen, Chicago Tribune, 21 June 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