I have several definitions for this phrase in English, here we go:

1.-To have fun.
2.-To do something different (E.G. Change your lifestyle)
3.-Often used as an instruction to someone who complains.
4.-Become aware or learn how.
5.-Get a well-balanced life (become happy, successful, etc.)
6.-Stop bothering me.

Now my main question is this, which of these meanings, apply more for the phrase "Get a life" and can you give an example of each one, if were the case, to understand better the phrase and the situation too.
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Thanks in advance,
Antonio.