のいないところで悪口を

talk badly about … behind their back

→ の
phrase Rare #45,358
talk badly about … behind their back The person before の is the one being talked about. This phrase is usually followed by 言う (to say) or a related form.
JA: 彼のいないところで悪口を言わないで。
EN: Don't talk badly about him behind his back.

人のいないところで悪口を言わないで下さい。

Please don't bad-mouth people behind their backs.

相手のいないところで悪口を言うな。

Don't badmouth people behind their backs.

その人のいないところで悪口を言ってはいけない。

You shouldn't speak ill of someone behind their back.

ジムのいないところで悪口を言ってはいけない。

You mustn't speak ill of Jim behind his back.