Acid

Acid is a corrosive threat to you, your pet, your metal weapons and armor and your enemies. It can come from monsters, comestibles, potions of acid ("This burns!"), and perhaps other sources.

Acidic monsters include acid blobs, gelatinous cubes, yellow dragons, and others. Acid resistance is granted by being polymorphed into acidic monsters or by wearing an alchemy smock; it cannot be intrinsically granted.