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)
"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.
There are many different ways of saying a veiled physical threat. Depending on different situation, the phrase maybe different. For example:
你什么意思？-- when the person do something making you annoying, and you want to stop them, just ask what they are doing, but with some emotions like this phrase, they would feel be threatened.
咱们走着瞧！-- when the person do something bad, and you want to struggle with them in a long term, you can say this to threat them, it's never end.
敬酒不吃吃罚酒啊？-- when you were doing something good to the person, but he/she refused the kindness. Then you may want to threat and punish them. Then goes for this one.