هذا جنون.

A common pronunciation is:

hādhā junūn

More carefully in fully vocalized MSA, it is:

هٰذَا جُنُونٌ
hādhā junūnun

A few notes:

• هذا = hādhā
• جنون = junūn
• In normal pause at the end of a sentence, the final -un is usually not pronounced, so junūnun becomes junūn.

Also, the ذ sound in هذا is like the th in this.

In Arabic, the verb to be in the present tense is usually omitted.

So هذا جنون literally looks like:

• this
• madness

But it naturally means:

• This is madness.

This is very normal in Arabic nominal sentences. You do not need a separate word for is here.

The sentence has two words:

• هذا = this
• جنون = madness, insanity, or craziness

So the structure is basically:

• هذا = the thing being pointed to
• جنون = what it is being described as

This is a very common question because learners are told that Arabic demonstratives agree with gender.

The short answer is:

• هذا is the masculine singular form of this
• هذه is the feminine singular form of this

Here, the predicate is جنون, and جنون is grammatically masculine, so هذا is the expected form.

Also, when you say this is X, Arabic often uses the demonstrative that matches the grammatical gender of what is being referred to or understood in context.

جنون is grammatically masculine.

That is why you would normally say:

• هذا جنون

not:

• هذه جنون

Remember that grammatical gender in Arabic does not always match natural gender. A noun like جنون is an abstract noun, but it still has a grammatical gender, and here it is masculine.

Because Arabic often uses an indefinite predicate in this kind of sentence.

So:

• هذا جنون = This is madness
• not necessarily This is the madness

The noun جنون here is indefinite and general. It describes the situation as an instance of madness/craziness.

If you added ال and said هذا الجنون, that would usually mean this madness rather than this is madness. So the meaning would change.

This is a basic nominal sentence in Arabic.

• هذا is the subject of the sentence
• جنون is the predicate

In Arabic grammar terms, you may see them called:

• مبتدأ for the first part
• خبر for the second part

So the pattern is:

• هذا = what you are talking about
• جنون = what you are saying about it

In fully inflected formal Arabic, it would be:

هٰذَا جُنُونٌ
hādhā junūnun

Why -un?

Because جنون is:

• indefinite
• in the nominative case here as the predicate of this nominal sentence

However, in normal speech and when pausing at the end of the sentence, the final -un is usually dropped in pronunciation, so you simply hear:

هذا جنون
hādhā junūn

Not with the same meaning.

Compare:

• هذا جنون = This is madness / This is insane
• هذا مجنون = This is a crazy man or This one is crazy if referring to a masculine person

Why?

• جنون is a noun meaning madness/insanity
• مجنون is an adjective or noun meaning crazy / insane / crazy person for a masculine singular referent

So هذا جنون comments on a situation or thing.
But هذا مجنون describes a masculine person or masculine referent as crazy.

Yes. It is a very natural exclamation in Arabic for reacting to something shocking, unreasonable, or extreme.

Depending on context, it can mean things like:

• This is madness.
• This is insane.
• This is crazy.

It is often used about situations, decisions, prices, behavior, or events.

هذا جنون is perfectly good Modern Standard Arabic and is also easy for Arabic speakers to understand outside strict formal contexts.

In everyday spoken dialects, people may use different expressions depending on the region, but this sentence is still clear, natural, and widely understandable.

So for a learner of MSA, it is a very useful sentence to know.