"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."