turn one’s back on someone—abandon or ignore smb.; refuse to support a person:
- When he seemed to turn his back on me I felt bereft, really. But then one day he rang up and I could hear his voice shaking with emotion.
when someone’s back is turned—do smth. when a person is not present or looking:
- If they’re saying this about Fred, what are they saying about me when my back is turned?
Note: Neither expression correlates in meaning with the phrase see the back of someone—be pleased when smb. leaves because one does not like the person:
- I wasn’t sorry to see the back of Suzy after what she did, but it really broke my heart to let Ziggy go.