Literally: May it be sacrificed for your head (Your head is much more valuable than whatever “it” may be.)
Meaning: Don’t worry about it. / Forget about it.
How to use it: You wrecked your dad’s car and are afraid he’ll get mad. Instead, he says, “Fadâye saret! You’re ok, that’s all that matters.” Or you’re a guest in someone’s house and break a vase. You feel awful. “Fadâye saret! I didn’t like that vase anyway,” they re****ure you.