Many people have said that this word means 'leave it out' or 'don't bother'. We it does mean this but it is also used as a sign of agreement to a situation.
It largely depends on
the tone
So it can either mean 'fuck that', 'can't be arsed' or on the other had 'yeah mate', 'lets have some of that'
If said in the negative term it would be said with a lowering in the
pitch of voice towards the end of the phrase, if however said in the positive it would be said with a rise in the pitch of the voice towards the end of the phrase.
As well as this in the positive it is accompanied by
the whip thing chavs do on the 'that'