"Three longs two shorts"
Used to refer to an unexpected misfortune, especially death
Negative
你要有个三长两短,我也不想活了 | Nǐ yào yǒu gè sānchángliǎngduǎn, wǒ yě bù xiǎng huó le
If anything should ever happen to you, I don't want to live
[Source]
万一你有个什么三长两短的,他这下半辈子就会生活在罪恶感中了 | Wàn yī nǐ yǒu gè shénme sānchángliǎngduǎn de, tā zhè xià bànbèizi jiù huì shēnghuó zài zuì'è gǎn zhōng le
If anything ever happened to you, he would live in guilt for the rest of his life
[Source]
If anything should ever happen to [a person]
一长两短 (yī cháng liǎng duǎn)
No one appears to be able to conclusively determine where this chengyu actually came from. I've listed the two most popular of the four stories that I've seen floating around. Both of them have to do with coffins.
Uncertain -- Either my resources all attributed this chengyu to different people or this information has been lost to history