when you are talking with someone (say a friend) and they get a vacant look (like they are thinking or something) and when you ask what's up they give the reply:

"Nothing much"

So irritating.... because common courtesy requires you to ask again... cause its nothing much! Not nothing at all.... and if you don't ask and your conversation partner turs out to be a girl, then its as if you don't care......

Another of the similar irritating expressions is "Not really" (when asked "are you thinking about something?")

The best part is that I love saying it.... but its meant to be a dismissive gesture.