- 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]