1755, "commission with jurisdiction over a port or river," from -cy + Latin conservant-, present-participle stem of conservare "to keep, preserve, keep intact, guard," from assimilated form of com-, here probably an intensive prefix (see com-), + servare "keep watch, maintain" (from PIE root *ser- (1) "to protect").
Earlier was conservacy "preservation under law, protection" (mid-15c., Anglo-French conservacie). Meaning "official preservation of undeveloped land" dates from 1859 (first reference is to protection of bo trees in Ceylon). General sense of "act of preserving" is by 1832. Meaning "institution concerned with the preservation of nature, undeveloped land, etc." is by 1949.