Fiona is sending a message in this song, which appears to be directed to a woman who is seeing a man she has a history with, warning her that he's bad news and keeping them apart.
Apple wrote it instead of reaching out to this person directly, which is often how she communicates. "I would write letters to my parents because they wouldn't listen to me. I would write letters so they'd have to be quiet until the letter was over, so they wouldn't interrupt me," she told Vulture
. "If I try to get in touch with somebody and talk through things, and they won't talk to me, then, sorry, I got to write a song."