phrase → that You use that's that to say there is nothing more you can do or say about a particular matter. [spoken]
⇒ 'Well, if that's the way you want it,' he replied, tears in his eyes, 'I guess that's
⇒ 'I want you to go home.'—'I'm staying here, and that's that.'
COBUILD Advanced English Dictionary. Copyright © HarperCollins Publishers