The Suffix -OUS in English: Meaning, Spelling and Examples

Learning how words are built can make English vocabulary much easier to understand. One particularly useful word ending is the suffix -ous in English, which is commonly used to form adjectives such as dangerous, famous, ambitious and mysterious.

Have you ever wondered why words such as dangerous, nervous, ambitious and mysterious all end in the same way?

The ending -ous is a very common English suffix used to form adjectives. Learning how suffixes work can help you recognise unfamiliar words, expand your vocabulary and choose the correct form of a word in speaking and writing.

This is particularly useful for learners preparing for B2 First and C1 Advanced, where word formation is an important part of Reading and Use of English.

In this lesson, you will learn what the suffix -ous means, how it is used, how spelling sometimes changes when we add it, and some of the most useful adjectives ending in -ous.

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What Does the Suffix -OUS Mean?

The suffix -ous is normally used to form adjectives.

It can often express meanings such as:

• having a particular quality
• characterised by something
• full of something

For example:

danger → dangerous

A dangerous situation is one that involves or may cause danger.

poison → poisonous

Something poisonous contains or produces poison.

mystery → mysterious

Something mysterious has a quality that is difficult to understand or explain.

However, it is better not to think of -ous as having one exact meaning in every word. In many cases, the safest approach is to learn the adjective together with its related noun.

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Common Adjectives Ending in -OUS

Dangerous

Meaning: likely to cause harm, injury or damage.

• Driving too fast can be extremely dangerous.

Word family: danger → dangerous → dangerously

Famous

Meaning: known by many people.

• The city is famous for its beautiful architecture.

Useful expression: be famous for something

• Cambridge is famous for its university.

Nervous

Meaning: worried, anxious or slightly afraid about something.

• I always feel nervous before an important exam.

Useful expression: be nervous about something

• He was nervous about speaking in public.

Ambitious

Meaning: determined to be successful or achieve something difficult.

• The government announced an ambitious environmental programme.

Word family: ambition → ambitious → ambitiously

Mysterious

Meaning: difficult to understand, explain or know much about.

• The police are investigating his mysterious disappearance.

Word family: mystery → mysterious → mysteriously

Poisonous

Meaning: containing poison or capable of causing illness or death if eaten, touched or absorbed.

• Some wild mushrooms are highly poisonous.

Word family: poison → poisonous

Envious

Meaning: wishing that you had something another person has.

• There is no point being envious of other people’s achievements.

Useful expression: be envious of somebody/something

Jealous

Meaning: unhappy because someone has something you would like to have, or worried that someone you love may prefer another person.

• She became jealous when her partner started spending more time with his new friends.

Although jealous and envious can sometimes overlap in everyday English, envious is more specifically connected with wanting something that another person has.

Miraculous

Meaning: extremely surprising and unlikely, especially when something very good happens.

• The passengers had a miraculous escape from the accident.

Word family: miracle → miraculous → miraculously

Courageous

Meaning: very brave, especially in a difficult or dangerous situation.

• The firefighters made a courageous attempt to rescue the family.

Word family: courage → courageous → courageously

Curious

Meaning: wanting to know or learn about something.

• Children are naturally curious about the world around them.

Useful structures: be curious about + noun

• She was curious about his new job.

be curious to + infinitive

• I was curious to see how the experiment would work.

Generous

Meaning: willing to give money, time or help freely.

• The company made a generous donation to the charity.

Word family: generosity → generous → generously

Humorous

Meaning: funny or intended to make people laugh.

• His speech was informative but also surprisingly humorous.

Word family: humour → humorous → humorously

Numerous

Meaning: many.

• The proposal has caused numerous problems.

This adjective is particularly useful in more formal writing.

Compare:

• There were many complaints.
• There were numerous complaints.

Both are correct, but numerous sounds somewhat more formal.

Obvious

Meaning: easy to see, understand or recognise.

• It was obvious that something was wrong.

Word family: obvious → obviously

Precious

Meaning: valuable, important or greatly loved.

• Clean drinking water is a precious resource.

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Spelling Changes with the Suffix -OUS

Adding -ous is not always as simple as putting three letters onto the end of a word. The spelling may change.

Words ending in -y

In some words, y changes before the suffix is added.

mystery → mysterious

glory → glorious

envy → envious

It is useful to learn these as complete word families rather than trying to apply one spelling rule to every word.

Words ending in -e

Sometimes the final e disappears.

fame → famous

But other words keep part of the original spelling:

courage → courageous

This is another reason why learning the whole word family is useful.

Nouns ending in -tion

Some nouns ending in -tion form adjectives ending in -tious.

ambition → ambitious

caution → cautious

superstition → superstitious

Notice that the spelling changes considerably.

Words Ending in -ious

A large number of adjectives end in -ious.

For example:

• ambitious
• curious
• mysterious
• obvious
• precious
• serious
• various

The i forms part of the spelling before -ous, so it is useful to recognise -ious as a common adjective ending.

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Quick Practice: Form an Adjective with -OUS

Complete each sentence using the word in brackets.

1. Swimming here can be extremely __________ because of the strong currents.
   (DANGER)
2. She is very __________ and hopes to become a senior manager before she is thirty.
   (AMBITION)
3. The police are investigating the __________ disappearance of a local businessman.
   (MYSTERY)
4. Some plants can be __________ if they are eaten.
   (POISON)
5. Everyone admired the firefighter’s __________ actions.
   (COURAGE)
6. The castle is __________ for its beautiful gardens.
   (FAME)
7. The organisation has received __________ complaints from local residents.
   (NUMBER)
8. I was __________ to know why she had suddenly changed her mind.
   (CURIOSITY)

Answers: 1. dangerous, 2. ambitious, 3. mysterious, 4. poisonous, 5. courageous, 6. famous, 7. numerous, 8. curious

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Final Thoughts

The suffix -ous is one of the most useful adjective endings in English. Once you learn to recognise it, words such as dangerous, mysterious, ambitious, courageous and poisonous become much easier to understand and remember.

Instead of learning each adjective on its own, try learning word families:

danger → dangerous → dangerously

ambition → ambitious → ambitiously

mystery → mysterious → mysteriously

This approach can help you expand your vocabulary, improve your writing and deal more confidently with word-formation exercises in B2 First and C1 Advanced.

Related posts: 

• Adjective Suffixes in English 
• Suffixes in English (Noun, Adjective, Verb & Adverb Suffixes) 
• Word Formation for B2 First
• B2 First Use of English
• C1 Advanced Use of English 
• Prefixes in English