a slang word used to stopsomeone from saying something negative about another person that isn't around to defend themselves.
Josh said "Wow, did you see her outfit, she looks like she just came from the dollar store and they didn't even have her size" Grant said you need to "Squash it and be nice!!"
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.