Metagaming

Metagaming as it applies to NetHack is simply the use of strategies that are widely considered to be 'cheap' 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 an online server

If playing on NAO or some other online server, asking another player about their inventory if you identify it on a bones level is considered a form of metagaming.

Price identification

Main article: Price identification

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