الخصر ضيق.

Because this is a nominal sentence in Arabic. In the present tense, Arabic usually does not use a separate word for is / am / are.

So:

• الخصر ضيق literally looks like the waist narrow
• but it means The waist is narrow

This is completely normal in Modern Standard Arabic.

If you wanted a past-tense version, then Arabic would use a verb, for example:

• كان الخصر ضيقًا = The waist was narrow

In Arabic, adjectives normally come after the noun they describe.

So:

• الخصر الضيق = the narrow waist

That is a noun phrase: noun + adjective.

But in your sentence:

• الخصر ضيق

the second word is not just part of a noun phrase. It is the predicate of the sentence, so the whole sentence means:

• The waist is narrow

So Arabic uses the same general position—after the noun—but the structure is different:

• الخصر الضيق = the narrow waist
• الخصر ضيق = the waist is narrow

Because here الخصر is the subject and it is definite: the waist.

The word ضيق is the predicate adjective: narrow. In this kind of sentence, the predicate is usually indefinite, so it does not take الـ.

That is why you get:

• الخصر ضيق = The waist is narrow

If you said:

• الخصر الضيق

that would no longer be a full sentence meaning The waist is narrow. Instead, it would mean:

• the narrow waist

So the presence or absence of الـ helps show the difference between:

• a sentence
• and a noun phrase

In fully vowelled, formal MSA, this would usually be:

• الخَصْرُ ضَيِّقٌ
• al-khaṣru ḍayyiqun

Here is why:

• الخَصْرُ ends in -u because it is the subject (mubtadaʾ)
• ضَيِّقٌ ends in -un because it is the predicate (khabar) and is indefinite

In normal writing, these short vowel endings are usually not written, so you commonly just see:

• الخصر ضيق

It is pronounced approximately:

• al-khaṣr
• in fully inflected speech: al-khaṣru

A few useful points:

• خ is like the ch in German Bach or Scottish loch
• ص is an emphatic s
• The cluster at the end, -ṣr, may feel unusual to English speakers

So the word is something like:

• khaṣr

with a strong, emphatic ṣ sound.

Here it is the adjective ḍayyiq, meaning narrow.

A careful pronunciation is:

• ḍayyiq
• with case ending: ḍayyiqun

Important detail: this is not pronounced like a simple ḍīq in this sentence. As an adjective meaning narrow, it is normally understood as ḍayyiq.

So the full sentence in careful pronunciation is:

• al-khaṣru ḍayyiqun

The doubled y sound is important.

Because it agrees with الخصر, which is treated as masculine singular.

In Arabic, adjectives agree with the noun in:

• gender
• number
• definiteness when they are attributive adjectives

Here, since ضيق is the predicate, it still matches the noun in gender and number:

• الخصر = masculine singular
• ضيق = masculine singular

If the noun were feminine, the adjective would usually be feminine too.

In this sentence, ضيق is functioning as an adjective meaning narrow.

Arabic words can sometimes belong to related patterns that create confusion for learners, especially when short vowels are not written. But in:

• الخصر ضيق

the natural reading is clearly adjectival:

• The waist is narrow

So grammatically, it is the predicate adjective of the sentence.

Yes, but that changes the structure and usually the style.

• الخصر ضيق = The waist is narrow
• ضيق الخصر = literally narrow of waist

The second one is a more descriptive expression and can function like:

• narrow-waisted
• one with a narrow waist

So the original sentence is the straightforward basic sentence pattern. The reversed version is not the same simple structure.

Yes. It is a complete, natural Arabic sentence.

Its structure is:

• الخصر = subject / topic
• ضيق = predicate / comment

This is one of the most common sentence types in Arabic: a nominal sentence with:

• a definite noun first
• then a predicate

So even though it is only two words, it is fully complete.

Yes, potentially both, depending on context.

الخصر means the waist, so the sentence could refer to:

• a person’s waist
• the waist area of a dress, shirt, or other garment

Arabic often leaves that kind of detail to context. The grammar stays the same either way:

• الخصر ضيق = The waist is narrow

The difference is definiteness.

• الخصر ضيق = The waist is narrow
• خصر ضيق by itself is usually understood as a narrow waist

So:

• with الـ, you have a definite subject and a full sentence
• without الـ, it is more likely a noun phrase rather than a complete statement

That is a very common and important distinction in Arabic.