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Suffix · forms adjectives

-proof

Forms adjectives meaning "resistant to" or "that cannot be penetrated or damaged by": bulletproof, waterproof, foolproof, soundproof, fireproof, childproof.

In Spanish: a prueba de / resistente aBasic

Written by Bryan López, English teacher · Updated June 2026

What does -proof signal?

-proof comes from "proof" in the sense of a test or trial. A "-proof" object has passed the test — it has demonstrated resistance to something: Physical resistance: • bulletproof = resistant to bullets • fireproof = resistant to fire • waterproof = resistant to water • soundproof = resistant to sound transmission • bombproof = resistant to bomb blasts • shatterproof = resistant to shattering Conceptual or social resistance: • foolproof = so simple or reliable that even a fool cannot make a mistake • childproof = designed so that children cannot open or operate it • tamper-proof = resistant to unauthorized interference • future-proof = designed to remain useful as conditions change

Foolproof: from mild insult to technical praise

"Foolproof" is one of the most common -proof compounds in everyday and technical English. At its origin it is mildly disparaging — a "foolproof" mechanism is so simple that even a fool cannot get it wrong. Today it is used without negative connotation as a synonym for "infallible" or "guaranteed to work": "a foolproof plan" = a plan that cannot fail "a foolproof method" = a method that always works "It's foolproof." = It cannot go wrong. Very frequent in technical instructions, advertising, and persuasive writing.

-proof as a productive modern suffix

-proof is one of the most productive suffixes in modern English — it attaches to new words constantly to create ad hoc adjectives: "hack-proof system," "recession-proof industry," "future-proof technology," "smartphone-proof wallet." This productivity makes it especially valuable for reading technical, marketing, and advanced journalistic texts in English. Once you know the pattern, you can decode any new -proof compound immediately.

How it is pronounced

-proof/pruːf/ · sounds like "proof"

Tap the button to hear how the ending sounds. Each word in the table has its own audio.

Examples

Base word
With -proof
In a phrase
  • bulletbulletproofThe limousine was fitted with bulletproof glass.
  • waterwaterproofMake sure your boots are waterproof before the hike.
  • foolfoolproofThe new system comes with foolproof instructions.
  • soundsoundproofThe recording studio walls were completely soundproof.
  • firefireproofThe safe was fireproof and resistant to temperatures up to 1000 degrees.
  • childchildproofAll medicine bottles must have childproof caps.
  • tampertamper-proofThe packaging is tamper-proof to ensure product safety.
  • futurefuture-proofWe need to invest in future-proof infrastructure now.

Common mistakes

waterproof against rain
waterproof

"Waterproof" already implies resistance to water — "waterproof against rain" is redundant. Simply: "a waterproof jacket," "it is waterproof."

a foolproof that always works
a foolproof plan / method

"Foolproof" is an adjective and needs a noun: "a foolproof plan," "a foolproof system." It cannot stand alone as a noun.

A trick to remember it

The "X-proof" pattern is extremely productive in modern English. When you read "recession-proof," "hack-proof," or "future-proof," the meaning is always "resistant to X" or "that cannot be damaged or affected by X." You can apply this pattern to almost any noun to create a new technical adjective.

Practise what you learned

Exercise 1 · Form the word

Fill in: "The jacket is ___ so you can wear it in the rain." (resistant to water)

Hint: Water + proof: resistant to water.

Exercise 2 · Pick the right one

"The instructions are foolproof." What does this mean?

Exercise 3 · Form the word

Fill in: "The recording studio walls are ___." (impervious to sound)

Hint: Sound + proof: resistant to sound transmission.

Frequently asked questions

What does the suffix -proof mean in English?

The suffix -proof forms adjectives meaning "resistant to" or "that cannot be penetrated or damaged by": bulletproof, waterproof, foolproof, soundproof, fireproof, childproof. In Spanish it usually maps to a prueba de / resistente a.

How do you pronounce -proof?

The ending -proof is pronounced /pruːf/ · sounds like "proof". For example, "bulletproof".

Can you give an example of a word with -proof?

"bullet" becomes "bulletproof". It is a typical example of the -proof suffix.

Other useful suffixes

  • -bound

    From the past participle of "bind": forms adjectives meaning "confined by," "heading toward," or "obligated by." Spellbound, earthbound, homebound, hidebound, snowbound, inbound.

  • -like

    From Old Norse "líkr" (similar): forms adjectives meaning "resembling X" or "characteristic of X." Godlike, dreamlike, lifelike, warlike, childlike, ghostlike, wavelike, businesslike.

  • -ward / -wards

    From Old English "-weard" (direction): forms adverbs and adjectives of direction or tendency. Inward, outward, forward, wayward, awkward.

Learn every English suffix

-tion, -ness, -ful, -ly, -able... every ending you need to understand thousands of words at once.

View all suffixes
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