How to give a thinly veiled threat? I want to know a phrase in Chinese that most native speakers would immediately understand to be a (thinly veiled) threat to their physical safety, very similar to "You had better watch your back" in English. I read this question, but I am now sure how subtle 好自为之 is. I also thought of 小心点儿 but that would depend heavily on context. Any phrases are appreciated.
To make a subtle threat in Chinese is the same as in English-- just imply a threat without actually using threatening words.

One frequently used method is  反話 (irony)

For Example: 

"終有一天，我會雙倍地償還你的恩典" (There will be a day, I doubly repay your grace) 


This so call 恩典 might mean 'a stab to my back' , and the payback might mean 'a bullet to your head', but it is just an implied threat, The court cannot use these words as evidence against me if some horrible thing happen to you later.