UK exist from European Union didn't have an impact on the political side only, but on language side as well. 2 informal words had been surfaced up after the recent referendum that paved the way to UK exit from EU.
BREXIT stands for Britain Exiting the European Union (EU).
The REMAIN campaign was trying to stay in the EU, while the LEAVE campaign wanted to abandon the EU, mainly because they were concerned about immigration and believed that immigrants from EU, especially Eastern European countries will take their jobs.
Those who were in the LEAVE campaign quickly realized the bad consequences of leaving European Union and they 'bregreted' it (= Bregret). So, a person who bregrets leaving EU is now called a Bregreter or Bregreteer.
A sheer example of a Bregreteer can be seen in this Youtube video.