صاحبتي لبست فستان اسود جديد في الشغل امبارح.

It can mean either one.

In everyday Egyptian Arabic, صاحبتي commonly means:

• my female friend
• my girlfriend

Context decides which one is meant. If the situation is neutral, many learners should think of it first as my female friend, but in real life it can definitely mean girlfriend too.

صاحبتي is very common in spoken Egyptian Arabic.

• صاحبتي = everyday, colloquial Egyptian
• صديقتي = more formal, more MSA-like, less conversational in many situations

So if you are learning natural Egyptian speech, صاحبتي is exactly the kind of word you want to know.

Because the subject is feminine singular: صاحبتي.

In the past tense, Arabic marks gender:

• لبس = he wore
• لبست = she wore

So صاحبتي لبست means my female friend wore.

It can mean either, depending on context.

In Egyptian Arabic, لبس / لبست can be used for:

• wore
• put on

In this sentence, English would usually translate it as wore, because the sentence is describing what she had on at work yesterday, not focusing on the moment of dressing.

Because Arabic does not have an indefinite article like English a / an.

So:

• فستان = a dress
• الفستان = the dress

That means فستان اسود جديد naturally means a new black dress.

Yes, in more careful spelling, you would often write أسود.

But in informal Egyptian writing, people very often simplify spellings and leave out hamzas. So:

• أسود = more careful/formal spelling
• اسود = very common informal spelling

This is normal in casual Arabic writing, especially dialect writing.

Because that is the normal Arabic pattern: noun first, adjective after.

So Arabic says, literally:

• dress black new

not:

• black new dress

This is completely normal in both Egyptian Arabic and MSA.

Because they have to agree with فستان, and فستان is grammatically masculine.

Even though a dress may feel like a feminine idea in English, Arabic grammar does not work that way. The noun فستان is masculine, so the adjectives are masculine too:

• فستان اسود جديد = masculine noun + masculine adjectives

If the noun were feminine, the adjectives would change too.

The given order is natural.

In Arabic, when there is more than one adjective, the order can be a bit flexible, but not always with exactly the same feel. Here:

• فستان اسود جديد sounds natural for a new black dress

If you say فستان جديد اسود, it is understandable, but it can sound a little less natural or shift the emphasis slightly.

So for learners, فستان اسود جديد is a good model to follow.

في الشغل means at work or in/at the workplace.

Breakdown:

• في = in / at
• الشغل = the work / work / the job

In Egyptian Arabic, في الشغل is a very common idiomatic way to say at work. The definite article ال is normal here. You do not need to translate it word-for-word as in the work in English.

امبارح means yesterday.

It does not have to come at the end. Time expressions are fairly flexible in Egyptian Arabic. For example, all of these can work:

• صاحبتي لبست فستان اسود جديد في الشغل امبارح
• امبارح صاحبتي لبست فستان اسود جديد في الشغل

Putting امبارح at the end is very natural.

Yes. Very normal.

This sentence starts with the subject:

• صاحبتي = subject
• لبست = verb
• فستان اسود جديد = object
• في الشغل = place
• امبارح = time

Subject-first word order is extremely common in spoken Egyptian Arabic. Also, because this is dialect, you do not see the case endings and tanween that learners often meet in MSA grammar books. This is one reason spoken Egyptian looks and feels simpler than formal Arabic.