"Ó pá, tu vens ou não?" What's the origin of "pá" as a vocative expression?

Question

Even though I understand that "pá" is a vocative expression and that it's also typical European Portuguese usage, I have some doubts about it:

1. What's the etymology of "pá"? Is it a short form for another word?
2. I've been to Portugal several times but I heard it only from a few persons who seem to use it all the time. Was this a coincidence or is it really unusual? Can it be taken as an indicator of social class or educational background?
3. Is it informal and unlikely to be heard during a lecture or formal speech?
4. Is there a region where one is more likely to hear it? (e.g. north x south)

Answer 1

My contribution as a native portuguese speaker from Portugal:

1. The word pá is believed to be short for rapaz. I myself remember hearing my grandfather saying paz which is an even better indication of the root word rapaz. Example: 'Ó paz, tu que estás a fazer? Não faças isso!' So, I can see rapaz being transformed into paz for easiness and later into pá.

2. There isn't any relation to educational background. You will hear anybody use it in certain contexts.

3. It is very informal. The doctor or professor will use it if they are talking to friends and colleagues, but not so much at work. But that being said, it also depends on the attitude and ways of the person: I have had university professors using pá during lectures and it does give a more friendly feeling and environment in the class.

4. Difficult to say. I believe you would hear it more in the north. Also, the people in the north are more down-to-earth so they use this sort of expressions (informality, closeness, intimacy, and the like) more often.