Hi Elmer. 'Mean' is an adjective and has a variety of meanings which are listed below. Apart from these meanings, it's a sentence spoilt kids say to their parents or to others when they don't get their way. Mature adults shouldn't have to use this childish expression.
Dictionary definitions of the word 'mean'.
1. low in quality, value, or importance; paltry; poor; inferior: now usually in negative constructions
2. Rate low in social status or rank; of humble origin
3. poor in appearance; shabby
4. ignoble; base; small-minded; petty
5. stingy; miserly; penurious
6. ☆ bad-tempered; vicious; unmanageable: said of a horse, etc.
7. ☆ pettily or contemptibly selfish, bad-tempered, disagreeable, malicious, etc.
8. ☆ humiliated or ashamed
9. ☆ Informal : in poor health; not well; ill; indisposed
10. ☆ Slang: difficult to cope with