For all you, I, they, etc. care
Definition 1 of 1
Pro Tip 1/3Emphasizing Indifference
Shows extreme lack of concern or interest towards someone else's choice or situation. 
Wear a clown suit to the meeting for all I care; your attire is your business.
Pro Tip 2/3Mild Dismissal
Indicates mild irritation or annoyance, suggesting the matter is trivial or unimportant. 
She can move to Mars for all I care; I'm tired of her drama.
Pro Tip 3/3Underlining Freedom
Implies that the person is free to make their own choices, without needing approval or consent. 
Take the last slice of pizza for all they care; they're not hungry anyway.