water off a duck's back
全然不起作用
A:How is Grandpa doing recently?
A:最近爷爷身体怎么样?B:Not good. The doctor told him not to smoke again, but it just rolls off him like water off the duck's back!
B：不好，医生让他不要再抽烟了，可这被他当成耳旁风，一点儿用也没有!
A:Maybe I would talk with him someday.
A:哪天我得跟他谈谈。B:I hope it will be of some use.
B：希望这会有点儿用。