probably originated in online posts at houseblogs.com or houseinprogress.net
PO (singular); POs (plural)
early 21st century
1 : people who owned a home before the current occupants
2 : a rhetorical device, called in to explain a weird, tacky, or disfunctional design or structural choice.
Typically, the term is used by home renovators or restorers in a derogatory way, as in "why would the previous owners put two layers of THE SAME wallpaper on the wall?* Simply to make wallpaper stripping a challenge?", or "I can't believe the POs put four giant screws in to hold up a tiny block of wood, thus ruining the plaster when said wood was removed".
crazy folks; poor designers; #!$&*$! POs; people too lazy to figure out how to do things the right way; see also "jerk", "nincompoop", "imbecile"
*Yes, this post was inspired by actual events. The POs (probably a couple of POs back) did put two layers of crappy blue wallpaper on one of the walls. As if it's paint, and you get better coverage with 2 coats? Not so much. So first you had to strip one layer off to wet the other - but it was an optical illusion, since you couldn't see where one layer had been stripped and the other hadn't. Fun stuff.