What's it to (someone)?
What does someone care? What business is it of someone? Usually said out of annoyance or anger. I pay my rent and my share of the bills—what's it to him how I spend my money otherwise? What's it to them what I do on the weekends? That's my time to do as I like!
What's it to you?
Inf. Why does it matter to you?; It's none of your business. (A bit contentious.) Tom: Where are you going? Jane: What's it to you? Mary: Bill's pants don't match his shirt. Jane: Does it matter? What's it to you?