Let’s learn how to use the English suffix PROOF today, we can use it to make new words and grow our vocabulary.
When proof is used as a suffix with nouns it means – protected against or not damaged by |
bulletproof – (1) designed to resist the penetration of bullets(2) a plan or idea that has no errors, it is guaranteed to succeed |
waterproof – not allowing water to enter |
fireproof – not catching fire |
babyproof – protecting babies from accidentally getting hurt (This is often used as a verb) |
foolproof – (1) for a device: protected, or designed to be proof against misuse or error (Even a fool can use it)(2) for an idea or plan: (Even a fool can do it) |
soundproof – preventing or constructed of a material that prevents, the passage of sound |
What is a suffix?
A suffix is a letter or group of letters added to the end of a word to make another word, such as -ly in quickly or -ness in sadness
https://www.oxfordlearnersdictionaries.com/definition/american_english/suffix?q=suffix
I made this vocabulary quiz for my Facebook followers a few years ago.

The answer is…
The windows of the president’s limousine are bulletproof.
bulletproof [adjective] designed to resist the penetration of bullets.
- Members of the swat team wear bulletproof vests when they work.
- Superman is bulletproof.
- The stores in the bad parts of town usually have bulletproof glass in their windows. I never go there!

We also use bulletproof to talk about an idea, plan, or concept. The plan has no errors, it is guaranteed to succeed.
- You’ve thought about this plan well, it sounds bulletproof.
- This method of cooking French toast is bulletproof.
Proof as an English suffix
The English suffix –proof is used with many other nouns to mean protected against or not damaged by
https://dictionary.cambridge.org/dictionary/learner-english/proof_2
Here are 5 more examples of nouns that we use with the suffix –proof
waterproof – Not allowing water to enter
“My new shoes are waterproof so I can wear them in the rain.”

fireproof – Not catching fire
“The bank safe is fireproof, so even if there’s a fire at the bank the money in the safe won’t get burned.”

More great SUFFIX practice
How to use the suffix EN (Increase vocabulary + VIDEO)
baby proof – Protecting babies from accidentally getting hurt (This is often used as a verb)
“After our son started walking we baby proofed our kitchen so he wouldn’t get hurt. The cabinets can’t be opened and we covered the electrical sockets.”

foolproof – For a device: protected, or designed to be proof against misuse or error. (Even a fool can use it)
For an idea or plan: (Even a fool can do it) source
“I just learned a foolproof way to cook French toast in the microwave! It’s easy and perfect every time.”

soundproof – adjective – preventing or constructed of a material that prevents, the passage of sound.
“I just bought some soundproof insulation to reduce the street noise in my living room.”

verb – make (a room or building) resistant to the passage of sound.
“Professional recording studios have soundproofed walls.”
Other posts about English suffixes: