The act of masterbation while watching a reflection of yourself preform the act. Most commonly this is purpotrated in a hotel room in front of a mirror.
*door opens*
Connor: So Craig what are you doing in...OH MY GOD! WHAT THE HELL?
Craig: I was, eh, um, er...
Connor: YOU WERE WANKING OFF TO YOUR OWN REFLECTION!
Craig: I was just...er...
Connor: That was a Valkenburg Mirror you sick...I MEAN FOR FUCKS SAKE WHAT YOU WERE THINKING?!
Craig: Um...
To take something small, that doesn't quite qualify as a theft. Probably from the Danish "skæv" or the Dutch "scheef", both of which are pronounced similarly, meaning "askew, or not quite right'. To change an item's ownership without permission, but only something small and of little worth.
"I skeefed an apple off the neighbor's tree." "I skeefed some chips outta your bag when you looked away." "Don't skeef my chair when I go to the bathroom."