I play a roguelike now and then and the term "savescum" comes up fairly often. What does it mean, and where did the term come from?
Savescumming generally refers to the practice of reloading a saved game prior to a random event, repeatedly, until a favorable outcome is achieved.
The term originated in the Roguelike community, as the genre is known for the frequent occurrence of perma-death as a feature - in that community, any use of saved games to recover from a fatal mistake or poor roll of the dice, rather than restarting the game entirely, is considered to be unsporting or 'scummy' behavior.
These days, the term is often used in a variety of genres to refer to abuse of reloaded saves to yield optimal results.
"Scumming" is a general term for performing an action repeatedly in a game in order to yield optimal results. It is originally from roguelike games, where you're typically expected to deal with an ever-increasing difficulty level as the game progresses. Any action you take in order to avoid this difficulty curve is considered scumming.
For instance, in many Roguelikes dungeon levels are randomly generated, and may or may not contain certain powerful items. If you were to repeatedly force a floor to be regenerated (by going up and down stairs between levels repeatedly), this would be called "stair scumming." Some roguelikes (notably Angband and its derivatives) actually have this as a feature - you can ask the game to repeatedly re-roll levels every time you go down a staircase until it generates an "optimal" or "unique" variant.
Save scumming in particular is the use of save files to achieve a "scummy" result - for instance, backing up your save right before opening a chest, and then if the chest doesn't contain the item you want, reloading the save and trying again.
Some games, such as Nethack, contain code that reject a save file if it is not the "original" copy of the file. Other games, such as XCOM: Enemy Unknown, store the random seed in the save file, so that if you save before performing a particular action, the result will be the same if you perform that action again after a load. Like all anti-cheat systems, these can be bypassed by a dedicated save-scummer.
It is the process of reloading a save for the purpose of redoing something to achieve what you perceive to be a more desirable outcome. The accepted exception to this is if a game crashes without you deliberately causing it to do so and you have to replay from the last legitimate on application close save which results in ending up at a different spot when you recover where you were. (This is why roguelikes would not usually delete your save after loading, but some very hardcore players will expect all games to be terminal--if you do not save and quit your save is gone end of story.)