Yep, It's Drywall

Me:  So I'm gonna knock out this wall right here to get rid of this stupid little bathroom and make one nice-sized bathroom, and then that tub you're givin me will go about right there where that sink is now.

Greg:  Wait a minute, are you sure that's not an original wall?

Me:  I'm sure.  Welllll....now that you say that...

Greg:  I'm not sure, hon.  It makes sense that this might've been a little room off the back porch originally, like a 19th-century version of a mudroom.

I get a hammer.  WHACK!!

We peer at the hole I've just knocked in the bathroom wall.

Greg:  Yep, it's drywall.