conservatory n.

late 14c., conservacioun, "preservation of health and soundness, maintenance in good condition, act of guarding or keeping with care," from Latin conservationem (nominative conservatio) "a keeping, preserving, conserving," noun of action from past-participle stem of conservare "to keep, preserve, keep intact, guard," from assimilated form of com-, here perhaps an intensive prefix (see com-), + servare "keep watch, maintain" (from PIE root *ser- (1) "to protect").

Meaning "preservation of existing conditions" in any sense is from mid-15c. The word has been used since late 15c. in reference to English municipal authorities who had charge of rivers, sewers, forests, fisheries, etc. Specifically with reference to preservation of nature and wild places by 1909.

The phrase conservation of energy apparently was coined in French by Leibnitz in 1692; it is attested in English publications from early 18c. as conservatio virum vivarum or partially nativized versions of it. The exact phrase is attested from 1853. 

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]