1. Usually used to describe, in a geographical sense, where a person is originally from.
The exact definition of Southern is up for debate.
Someone from the very north of England is likely to consider anyone who lives south of Yorkshire to be 'a southerner'.
2. The term can also be used to insult and stereotype a person who is from the south. Other common insults include southern fairy, soft southerner, southern pansy and posh southerner. These insults essentially mean the same as one another. The term 'southern' is particularly useful in insulting people who are 'soft', posh, unable to drink alcohol without falling over/vomiting and who need to wear a coat when the weather is a bit nippy.
'Southern' is commonly used as an adjective to describe the attributes held by someone from the south. The term is usually directed by someone from the north to someone from the south.
1. He is Southern as he was born in London/ Essex/ Manchester.
2. When a person from the south is shivering and people from the north are wearing t- shirts the northern people may use the phrase 'you are so southern'.
Similarly, if a southern person drinks too much (this is likely to be early in the evening when they are drinking with people from the north) and does something silly/ embarrassing the northern people may attribute the southern persons drunken state entirely to being 'southern'.