A useful NetHack tool. An oilskin sack is like an ordinary sack, with the important difference that it is waterproof. A NetHack player learns from bitter, bitter experience how important it is to have an oilskin sack -- either that, or how important it is to avoid water like the plague. Oilskin sacks are particularly important for Wizards, as spellbooks get erased from getting sprayed or dunked, and without spells, a Wizard is a just a weakling humanoid.

If only the oilskin sack was also a bag of holding! Sadly, it isn't -- the usual method therefore is to put your bag of holding inside the oilskin sack. This adds a pesky layer of nesting, but it's worth it for the combined item storing and waterproofing capabilities. Good luck finding one.