weaken

weaken — verb

1. to lose physical strength, power, influence, effectiveness, or determination; or

1.verbtransitive / intransitiveB2
Definition

to lose physical strength, power, influence, effectiveness, or determination; or to cause someone or something to lose these qualities.

Example

The flu left Minhô feeling tired and weakened his immune system for weeks.

transitive use: weaken + noun phrase (body/system)

Heavy rain weakened the old dam, so the engineers had to reinforce it quickly.

transitive, cause + effect structure with physical object

Common collocations
  • weaken someone's position
  • weaken the economy
  • weaken the immune system
  • weaken someone's resolve
  • weaken a structure
  • weaken a currency
Synonyms
  • undermine

    suggests gradual, often hidden damage to someone's position, authority, or confidence

  • erode

    emphasises a slow, continuous wearing away, like the erosion of trust or value

  • diminish

    focuses on reduction in size, importance, or intensity, often less dramatic than weaken

  • debilitate

    more formal and stronger; means to make very weak, especially physically or medically

Antonyms
  • strengthen

    direct opposite: to make or become stronger

  • reinforce

    to add extra strength or support to something, often physically or structurally

Grammar Patterns

weaken + noun phrase (transitive)

weaken (intransitive, no object)

Usage Note

Frequently used in both active and passive voice. The transitive pattern (weaken + object) is common for physical or structural subjects; the intransitive pattern (subject + weaken) is common for abstract subjects such as confidence, resolve, or arguments.

Common Mistakes

The medicine weakens the pain.
The medicine reduces the pain.
'weaken' means to make less strong, not to relieve pain. Use 'reduce' or 'ease' for pain.
His immune system weakened by the virus.' (missing auxiliary)
His immune system was weakened by the virus.
When the agent is mentioned after 'weaken', use passive voice ('was weakened by'). For intransitive use without an agent, say 'His immune system weakened.'

Word family

Synonym discussion

Sense: verb/1

All five words describe a loss of strength or effectiveness, but they differ in manner and scope. Weaken is the most general term — it covers physical strength, structural integrity, arguments, confidence, and economic power. Undermine implies gradual, often hidden damage, especially to authority, trust, or a position, as in 'the rumours undermined the manager's authority.' Erode suggests a slow wearing-away process, commonly used for abstract things like trust, patience, or value over time. Diminish focuses on a reduction in size, importance, or intensity — it is less dramatic than weaken and works well for status or reputation. Debilitate is the strongest and most formal; it means to make extremely weak, usually in a physical or medical context, as in 'the disease debilitated the patient for months.' Use weaken as your default, and pick the more specific word when the manner of weakening matters.

weaken ↗undermine ↗erodediminish ↗debilitate

Common collocations

Nouns

Adverbs

Phrases

  • weaken the case for
  • weaken someone's hold on

Prepositions

Etymology

Old NorseMiddle EnglishEnglish

Formed in Middle English from the adjective 'weak' (from Old Norse 'veikr', meaning 'soft, pliant, easily bent') plus the verb-forming suffix '-en', which means 'to make or become'. The verb replaced the older form 'weaken' that already existed in Old English from a different root.