This is a phrase based on the iconic Doofenshmirtz phrase, "Wow, if I had a nickel for the every time I was doomed by a puppet, I'd have two nickels. Which isn't a lot, but it's weird that it happened twice, right?" The phrase is not only a shortening of the phrase, but makes it easy to add to a conversation without jumpcutting to "haha funny reference".
Dude, this girl fell in love with me, dumped me, forgot she dated me, fell in love with me again, and dumped me AGAIN! Sigh, you know what they say about nickels and strange occurences...
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.