Acid resistance

From Wikihack

Acid resistance will protect you against acid damage, which is, however, rarely encountered and which does little damage. It does not protect your equipment against acid – you can do that by rustproofing it.

Acid resistance cannot be obtained by eating corpses. You can get it by wearing yellow dragon scale mail (or scales), or an alchemy smock. You could also polymorph into a monster that has intrinsic acid resistance.