Having an origin in football sporting rivalry, a word that describes a prominent element of British pub culture. Usually in a group of people over a drink the objective is to slander and belittle one or several of the people to try and impress and entertain others in the same group. The sharpest, wittiest and quickest person that is the most successful in belittling and offending others is considered elevated in the perceived pub social heirarchy.
Commonly referred to as "piss-taking", or "sledging", and one of the English sins featured in a recent British Television documentary series titled "The Seven Sins of England".
The Celtic supporter left the pub because he could not tolerate the slaggish behaviour of the Rangers fans.