Its a derogatory term used to describe that un which is attempted to be explained yet is unexplainable, therefore such concept is, was, and always will be such unless, except in the possible yet unlikely event of change of manner in form used as of by target individual applied to that of other means and/or fashions of original constructed inquiry.
When some resigns or retires from a job, they are on "eternity leave". This can also refer to someone who has passed away, especially someone who worked somewhere for a long time, retired/resigned, then passed away later.
Oh I'm sorry, you didn't know that Kelly is on eternity leave? She resigned and moved to Hawaii!
With regrets I would like to inform you that my boss is officially on eternity leave. I will never see or hear them again. Ever.