Although this sounds very strange indeed, this is a valuable NetHack intrinsic that prevents you from choking and inhaling noxious gas. It is acquired by wearing (or eating) an amulet of magical breathing or by polymorphing into an acid blob, an elemental, a vampire bat, a gargoyle, or one of jmvyFLMPVWXZ' (none of those things need to breathe).

Oddly, unbreathing will not protect you from drowning or strangulation -- either the DevTeam forgot, the DevTeam doesn't want this intrinsic to be too useful, or that blasted DevTeam is way ahead of me and realizes that death from strangulation usually occurs not from asphyxiation but from interrupted blood flow to the brain. Even if you don't need to breathe, presumably your body still needs oxygen, so complete submersion in water would kill you anyways. Or maybe I'm giving them too much credit on this one.