What is a hobnobber?

Answer 1

Shakespeare only once uses anything close to the word "hob-nob", in Twelfth Night when Sir Toby says in Act 3 Scene 4:

his incensement at this moment is so implacable,

that satisfaction can be none but by pangs of death

and sepulchre. Hob, nob, is his word; give't or take't.

The comma is there in the Folio text, showing that these are two separate words, not the single word "hob-nob". One text I have glosses this as "have" or "have not" (hob=have, nob=have not). The OED glosses the text similarly. Whatever the origin of this somewhat obscure phrase, it is clear that one of its meanings was a gamble, an "all or nothing" venture. This would appear to be the sense here: Sir Toby is telling Viola that Sir Andrew is not the kind of man to compose a quarrel; he will fight and risk his life on the all or nothing gamble of a duel.

Answer 2

A HobNobber is a cool and awesome geek.

Which is awesome if your a geek.(a nerd that is actually cool(:)

Answer 3

To mix socially, especially with those of a higher social status.