Grinding is a general term for doing boring stuff that proves to be of some use later on. It is well-described on wikipedia.

In ADOM, some activities which can be considered as grinding:

  • Precrowning via live sacrifices. Gives a random artifact without using much (if any) of your resources, but takes significant real-life time, since many monsters need to be led onto the altar.
  • Camping and killing monsters in cavernous levels like the Big Room.
  • Farming herbs.
  • Concentrating on using lots of time-consuming skills like Pick Pockets and Smithing.

The line between grinding and scumming is rather thin; in general, the latter abuses some bugs or unintended interactions, while the former is just investing a lot of real-life time for minor (compared to time spent - several precrowns and massive smithing can be game-breaking with enough luck and persistence) gains on the PC.