Why there is no article in 'I was out of town'

I have heard "Out of town" idiom when people say that they were away.

I wonder why there is no indefinite or definite article before the word 'town'?

IMO, it should be the definite article because I am talking about the particular town.

Answer

Chalk it up to idiomatic English. It's curious how we would generally say:

I was out of the area.

but:

I was out of town.

The same goes for the phrase away from:

I was away from the area.

I was away from town.

The same holds true for after we get back:

I'm glad to see you're back in town.
I'm glad to see you're back in the area.

When it comes to the word town, sometimes an article is optional:

I've been looking for you all over town!

I've been looking for you all over the town!

Both of those are acceptable, but I think you'll find the first one is more common and idiomatic.

Interestingly enough, I would never omit the article with the word city:

I was out of the city.
I was away from the city.
I'm glad you made it back to the city.
I've been looking for you all over the city.