A fancy term to describe all of the random bad stuff that happens to people every day for no real reason.
The phrase has two major applications - in English and Anglo-Australian Law it is used by judges to impress people and describe the unknown factors when determining compensation. In religious circles it is used to describe the hardships and trials that Jesus/Allah/Buddha/Satan can help you to overcome.
As an everyday non-judicial non-zealot you can still use it as a great excuse for why you did/didn't do something you shouldn't/should have. So next time your partner asks where the car keys are you can explain that they are lost due to the vicissitudes of life (also works well for homework and IT projects.