( informal )
1.
used to tell sb that sth is not important and that they should not worry about it :
'I still owe you for lunch yesterday.' 'Forget it.'
2.
used to tell sb that you are not going to repeat what you said :
'Now, what were you saying about John?' 'Forget it, it doesn't matter.'
3.
used to emphasize that you are saying 'no' to sth :
'Any chance of you helping out here?' 'Forget it, I've got too much to do.'
4.
used to tell sb to stop talking about sth because they are annoying you :
Just forget it, will you!