pay someone back in their own coin

retaliate by similar behaviour

以其人之道，還治其人之身

to coin a phrase

said ironically when introducing a banal remark or cliché

套用一種說法

I had to find out the hard way—to coin a phrase.

套句老話說，我是費了九牛二虎之力才搞清楚的。

the other side of the coin

One of two differing or opposing views or sides.

反面，反面意見：兩個不同的或相反的觀點和方面之一