respiratory adj.

late 14c., respiracioun, "act or process of breathing, inhalation and exhalation of air by the lungs," from Latin respirationem (nominative respiratio) "breathing, respiration," noun of action from past-participle stem of respirare "breathe again, breathe in and out," from re- "again" (see re-) + spirare "to breathe" (see spirit (n.)). Extended to plants by 1831. Milton used it for "act of returning to life" ("breathing again").

adjective and noun suffix, "having to do with, characterized by, tending to, place for," from Middle English -orie, from Old North French -ory, -orie (Old French -oir, -oire), from Latin -orius, -oria, -orium.

Latin adjectives in -orius, according to "An Etymological Dictionary of the French Language," tended to "indicate a quality proper to the action accomplished by the agent; as oratorius from orator; laudatorius from laudator. The neuter of these adjectives was early employed as a substantive, and usually denoted the place of residence of the agent or the instrument that he uses; as praetorium from praetor; dormitorium from dormitor; auditorium, dolatorium.

"These newer words, already frequent under the Empire, became exceedingly numerous at a later time, especially in ecclesiastical and scholastic Latin; as purgatorium, refectorium, laboratorium, observatorium, &c." [transl. G.W. Kitchin, Oxford, 1878]