Man has two senses, both of which go back to the very earliest uses of the word in OE.
- The human species or, when used with a determiner, a member of that species.
- An adult human male.
The species sense appears to be etymologically older; but the secondary sense of 'adult male' has by now become primary. OED 1 writes:
b. In the surviving use, the sense ‘person’ occurs only in general or indefinite applications (e.g. with adjs. Like every, any, no, and often in the plural, esp. with all, any, some, many few, etc.); in modern apprehension man as thus used primarily denotes the male sex, though by implication referring also to women.
The gradual development of the use of the unambiguous synonyms body, person, one, and (for the plural) folk(s), people, has greatly narrowed the currency of man in this sense; it is now literary and proverbial rather than colloquial.
Since OED wrote this, the success of the women's movement since the 1970s has tended to marginalize the human sense of the term even farther.
Nonetheless, in ordinary speech man is still inherently ambiguous. Consequently, except for those “literary and proverbial” situations OED mentions, we tend to avoid man altogether, employing male instead when we are trying to express what is proper to the male as opposed to the female sex.
Note, too, that even if that were not the case, man would not be an appropriate modifier for reproductive system or a baldness pattern, since these are characteristic of all males, not merely adult males.
Context is everything. When you are in doubt whether to use male or man, look at what terms you are contrasting with, whether implicitly or explicitly.
If you are contrasting humans with nonhumans, man is appropriate: “It’s not a fit night out for man or beast”.
If you are contrasting adult male humans with sub-adult male humans, man is again appropriate: “He is a man now, no longer a boy.”
But if you are contrasting male humans with female humans, male is appropriate: male strippers, nurses and college students are distinguished from the female strippers, nurses and college students who constitute (in the US, at least) the majority in those occupations.
The main difference in usage is between physical pain and emotional pain. "I am in pain" suggests there is ongoing pain, usually physical. "I am hurting" suggests there is ongoing pain, usually emotional.
This is an example of sense 2b the Merriam-Webster Learner's Dictionary:
2 a [+ object] : to make (someone) sad or upset : to cause (someone) to suffer emotionally
- Their lack of interest in her work hurt her deeply.
- You're only hurting yourself by holding a grudge against them.
- It hurt me to see her go.
b [no object] : to feel emotional pain or distress
- My sister has really been hurting [=has been very upset and unhappy] since her boyfriend left her.
And as for whether it's "slangy", here's a quote from a press conference Barack Obama gave in Nov 2014:
... there are still a lot of folks out there who are anxious and are hurting and are having trouble making ends meet, or are worried about their children’s future. [source]
There are hundreds more examples on www.americanrhetoric.com of speeches and interviews with similar examples of many different people using that word the same way.
Best Answer
The noun phrase following an interjection such as 'hello' restricts the intended audience, thus 'hello, boys and girls,' is appropriate when the speech is intended for the children, but adults are also present. 'Hello, ladies and gentlemen' means that you are not addressing the waiters, etc. Otherwise, 'hello everyone' is just as good.