وداعا، صديقي.

A careful MSA pronunciation is wadāʿan, ṣadīqī.

A rough English-friendly guide:

• wa-DAA-an
• sa-DEE-qee

A few sounds are tricky:

• ع in وداع is a deep throat sound with no exact English equivalent.
• ص in صديقي is a heavier, darker s than normal س.
• ق is a deep q sound, farther back in the mouth than English k.

It is perfectly good Modern Standard Arabic.

However, وداعًا sounds rather formal, literary, or emotional. In everyday spoken Arabic, people often use other goodbye expressions instead, depending on the dialect, such as مع السلامة in many contexts.

So this sentence is natural in MSA, but not the most casual everyday spoken goodbye.

It often feels closer to farewell than to a casual bye.

It can sound:

• formal
• heartfelt
• final
• literary

So وداعًا، صديقي may sound more emotional than a simple everyday parting.

The fully vocalized form is وَداعًا.

That final -an is called tanwīn fatḥ. In formal grammar, it marks the word as an indefinite accusative form.

In normal Arabic writing, short vowels are usually omitted, so learners often see وداعا instead of وَداعًا.

So:

• وداعا = the usual unvowelled spelling
• وَداعًا = the fully vocalized spelling

It is made of two parts:

• صديق = friend
• ـي = my

So صديقي literally means my friend.

The ending ـي is a possessive suffix, and it is pronounced as a long -ī sound.

Because a noun with a possessive suffix is already definite.

So:

• صديق = a friend / friend
• صديقي = my friend

Once you add ـي meaning my, you do not also add ال.

You can say it either way.

Both are natural:

• وداعًا، صديقي
• وداعًا يا صديقي

Adding يا makes the direct address more explicit, like Goodbye, O my friend / Goodbye, my friend.

Without يا, the sentence still works very naturally, especially in writing or in a slightly literary style.

You would say:

وداعًا، صديقتي

This is ṣadīqatī:

• صديقة = female friend
• ـي = my

So Arabic shows the gender of friend here, unlike English my friend, which can refer to either a man or a woman.