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!"
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."
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 reallya virgin, but he's an erstwhile virgin.