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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 August 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 formed

Noun + proof.

water + proof
waterproof
fire + proof
fireproof
fool + proof
foolproof

Fairly productive in modern English: almost any threat noun can combine with -proof (bulletproof, childproof, tamper-proof).

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

"-proof" = protected against, like a shield. Picture a shield blocking something specific: water + proof, bullet + proof.

Practise what you learned

Exercise 1 · Form the word

Exercise 2 · Pick the right one

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

Exercise 3 · Form the word

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.

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