be a peach

To be wonderful or sweetly helpful. Sometimes used as a condescending imperative. You brought me coffee? Ah, you're a peach. Thank you much for picking up the cupcakes on your way to the party—you're a peach. Hey, Sally? Be a peach and go fetch me a latte from the coffee shop on the corner.

you're a peach

You are very sweet or helpful. You brought me coffee? Ah, you're a peach. Thank you much for picking up the cupcakes on your way to the party—you're a peach.