Oxbridge slang for someone who is 'superkeen'; that is, keener than keen. Associated with a strong go-getting attitude, but one that is obvious and public, to the point of being awkwardly endearing or backfiring.
Unclear where 'nadster' originates from. Likely related to nads, as in someone who's "got balls", or the Polish 'nad', meaning 'above' - say, 'above keen'.
"Let's try and be first into the club!"
"Nah, nobody's in there. We'll end up floating around like abandoned toilet paper. Don't want to be super-keen nadsters."
"I looked at the slogan I'd painted up on the banner: 'Come on lads, be a nads(ter)!!'. What was I thinking? The pun didn't even make sense. I looked like such a super-keen nadster."