（求词源）
三长两短 is a chengyu that means "unforeseen disaster". It can also be a euphemism for death.
Here's an example of its usage:
明·罗贯中《三遂平妖传》：“万一些后再有三长两短，终不能靠着太医活命。”
What is its etymology though? One way of literally interpreting this chengyu is that it refers to three long things and two short things; what could they be?
There's a theory that it's referring to the coffin box, which is made up of three long and two short pieces of wood, but this seems like a false etymology.
Theory 1: A coffin is made of three long and two short pieces of wood. And a coffin means death, dangerous.
You also can find some clues in the wikipedia article about "使用筷子禁忌".
Theory 2: Taoist forecasts through burning incenses. It is the most dangerous condition that there are three long incense sticks and two short incense sticks after burning five incence sticks in some time.
Theory 3: In ancient times, a master swordmaker called Ouyezi (欧冶子) made five sharp swords. Three of them were long, the others were short. And an assassin used the one called "鱼肠剑" among the five swords to kill the ennemy King. So "三长两短" refers to coming across a dangerous accident.
Maybe the answer is this
- 学者以为三长两短指的是未盖上盖儿的棺材，因为棺材正好由三块长木板、两块短木板构成一个匣子。棺材是用来装死尸的，“三长两短”在口语中常指意外的灾祸、有生命危险的事故，或许与此有关。
The coffin has three long wood pieces and two short wood pieces, so 三长两短 is a sign for bad things
or in Chinese 三 does not mean really numbers, it refers to huge number of things(just like 三人行必有我师), so perhaps 三长两短 has something to do with it.
三长两短 refers to a coffin. The lower part of the coffin is made of 3 long wooden board and 2 short wooden piece of board. It is also referring to death or something very bad that related to life danger.
Other usage on non-living things, it refers to severe damage， unrecoverable malfunction or lost.
Example: 我可以把手机借给你，但是万一它有什么三长两短，你可要赔一部新的给我。
Translate: I can lend you my mobile, but if it is lost or damage, you have to replace it with a new one for me.
3 long 2 short can mean life is always up and down. Sometimes good, sometimes bad... so nothing to worry about... There'll be good times and bad times - it's interchangeable. Like the stock market: bulls & bears, economy: boom & bust, war & peace, etc... So it's a warning implied that don't expect things to run smoothly forever. Be aware of the unexpected hiccup.
According to here https://idiom.wlps.kl.edu.tw/dict_idioms/448.html
there are multiple variants of this word:
兩短三長, 一長半短, 一長兩短, and 三長四短 etc.
長 and 短 could mean some change while 三, 兩 or other numbers may be used to express that the amount of change is random and unpredictable. Based on my experience this word is often used to suggest the possibility of usually bad things that may happen in the future.
I burn 3 incense to my ancestor but one of them did not burn. I am very sure that i burned it throughly before placing for offering . But later on , one incense did not burn up. What does this indicate?
