Jo's expression barely shifts at the raise of that hand, even though her gaze goes to it and then back to Geralt's face. A slight bit more frown forms at realizing he doesn't know that answer, and it's a big, times insanity, leap. It feels both too insane and just insane enough to be true when it comes to Winchester Bullshit.

"No." She shook her head. "That's a guess, and an insane one at that."

She says it like maybe it will dispel the growing consideration. Or the fact she hates that there aren't computers here to look it up. Or the easy, straightforward truth, that Dean knows the answer, but Dean isn't here, because something is happening bad enough with Dean he isn't invited to the discussion.