Breath Weapon is a /tg/ slang term used to refer to any mystical or para-biological attack that is expelled from the mouth. This is typically in the same mythological vein as the iconic dragon breathing fire across its enemies. Popularized by Dungeons & Dragons, whilst the term is most associated with dragons, it has become usable across a wide variety of monsters and even some races.
Though the term most calls to mind actually breathing, it is used as a catch-all term for any kind of oral-based attack revolving around some form of hostile expulsion. Spitting gobbets of petrifying slime, vomiting corrosive sludge, screaming so loud that heads burst and belching with enough force to hurl an armored knight through the air are all covered by the term "breath weapon".