This is slang from the UK although it is used in Australia as well and can be used in number ways:
1 To make a foolish statement;
2 To talk nonsense or rubbish;
3 To say something which is quite
obviously false;
4 To
exaggerate your abilities or
knowledge of a subject;
5 To bluff or boast about something;
6 To be a self-aggrandising twat;