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."
Offbeat, gender-neutral way of referring to one's ex-wife or ex-husband.
I have a special ringtone in my cell phone that's only assigned to my ex-wife's phone numbers. So, when my cell phone makes that "oink-oink-oink" sound, I say "Oh, it must be my erstwhile spouse who's calling."
One who has had some but very little sexual contact with another person. Coined by F. C. Ware.
Yeah, some girl gave him some head when he was in college, but he's been living with his mother for the last 15 years. He's not really a virgin, but he's an erstwhile virgin.