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1.
previous, no longer there
Our erstwhile friend died Friday. He used to be your friend, but he died.
by Dessicator April 28, 2003
 
2.
"Former," "previous," "ex-"

Erstwhile works a lot like "ex," except it's an adjective instead of a prefix, much more formal and a little old-fashioned. Not everyone knows what it means. Lots of people slip it into sentences because it sounds fancy and makes them feel like they're from the 1890s.
My erstwhile and estimable business partner will be joining us for lunch."

"Wait, you guys aren't working together anymore?"

"What? No, we still are."

"Then why'd you say 'erstwhile'?"

"Because he went to Harvard, man! It doesn't get more erstwhile than that!"

"That's not what that word means, dude."

"Don't say 'dude.' It'll make you sound uneducated."
by DRF December 16, 2013
 
4.
See dictator
All erstwhiles should be deposed from their abused positions of power.
by Joe Cool February 15, 2003