Many times I have heard it while natives are speaking, specially Americans. I can not recall the contexts, perhaps they are very fast speaking. I wonder if any body here could give some hints in some contexts and provide some authentic examples they use in their speech evey day. Many thanks in advance.
Be about [to VP] is a semi-modal expression, like be going [to VP] or be able [to VP]. It signifies that VP is expected to happen in the very near future, and that the discourse concerns that expectation. How near will depend on context.
John was about to sit down to dinner when the phone rang. ... This suggests that John was actually moving toward the dinner table and the action was interrupted by the phone ringing.
Rumours that Microsoft is about to announce Office for iPad were current on the internet, in exactly those words, for at least three weeks before the actual announcement.
I am about to start graduate school. ... This might be said weeks or months before you actually plan to start graduate school. It suggests that you have been accepted by a school and your discourse will involve preparing to start.
As Bob Rodes points out, about here has the sense, “nearly, almost”. In other contexts it acts as an adverb: I am about ready = “I am almost ready”; but in the idiom be about [to VERB] it cannot be paraphrased this way: *I am almost to go is not English. And in the idiom it never has other main sense of about = “approximately, around”.
It's similar to "I am going to", but it means that I am going to do something in the very near future, that it is probably the next thing that I am going to do. For example, "I'm about to leave" has a similar meaning to "I'm on my way out the door".
You will often hear it in relation to an adjective as well: "Are you about ready to go?" Also, you will often see "just" as a way of adding emphasis. "Relax, I'm just about ready."
If you look at the sentence "It is about two o'clock", that means that the time is close, or in the vicinity of, two o'clock. This is closer to the standard meaning of the word about, and may give some insight into why we use the preposition to give the meaning about which you are asking.
‘To be about to’ is used for actions that will begin very soon (in relation to the narrative ‘now’.) Some examples in the present tense:
The train is about to leave.
Hurry up, it’s about to rain!
The students are yawning. It looks like a few are about to fall asleep.
And in the past tense:
I was about to call him when he walked in the door.
I was about to have dinner when the phone rang.
Contrast the latter with ‘I was having dinner when the phone rang.’ In this case dinner has already begun, but with ‘about to’ it has not.
A similar construction is ‘to be on the verge of’. The contexts in which each is used are not identical though – ‘verge’ may suggest that something is building up to an impending climax.
I find this job so frustrating that I’m on the verge of quitting.
The company is in bad shape. In fact, it’s on the verge of going bankrupt. ['of bankruptcy' also possible]