Metagaming

From Wikipedia: Metagaming is a broad term usually used to define any strategy, action or method used in a game which transcends a prescribed ruleset, uses external factors to affect the game, or goes beyond the supposed limits or environment set by the game.

Metagaming as it applies to NetHack is simply the use of strategies that are widely considered 'cheap', or by using external information to assist yourself. Some examples of metagaming:

Water Demon Hunting There is a small chance (1/30) of a water demon appearing when you quaff from a fountain. There is then a very good chance (80+DL/100) that the demon will immediately be hostile (100% chance if you have the Amulet of Yendor). The rest of the time, however, you get a free wish.

Asking About Bones on NAO Asking another player about their inventory if you identify it on a 'bones' level is considered a form of metagaming.

Price Identifying Using a shop to identify similar item types (oil lamp vs. magic lamp, for instance) is often practiced, but is a form of metagaming.