The loanword which is seemingly a Chengyu — 歇斯底里
Here's an amazing fact about a loanword in Chinese which is casually thought by many people to be a Chengyu. The word is 歇斯底里(xiē sī dǐ lǐ). Even by appearance, we would recognise it as a Chengyu, not to mention that it looks seemingly like a concise idiom just like all other Chengyu.
However, the word actually came from "hysteria", which is a specialised medical term for a mental disease that makes the patients experience extreme and uncontrollable emotions. It's hard to tell whether the word is first translated as a technical term or not, but the corresponding Chinese transcription, 歇斯底里, did have been used for the same disease before.
What is the most amazing about this word is, despite it IS a medical term used in specialised context, it seems to appear in casual usage much more frequently. People use it just like some casual or informal Chengyu and idioms, which has somehow turned 歇斯底里 into a daily idiom as well. Usually, 歇斯底里 is used to describe a mental condition of a person which makes the person have an extreme feeling that stirs him unnaturally, often with extremely uncontrollable emotions or behaviours.