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:
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.