advisory (adj.)

late 13c., avisen "to view, consider" (a sense now obsolete); late 14c., "to give counsel to," from Old French aviser "deliberate, reflect, consider" (13c.), from avis "opinion," from phrase ço m'est à vis "it seems to me," or from Vulgar Latin *mi est visum "in my view," ultimately from Latin visum, neuter past participle of videre "to see" (from PIE root *weid- "to see"). The unetymological -d- is from 16c. Related: Advised; advising.

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]

1610s, "one who gives advice," agent noun from advise (v.). The meaning "faculty assigned to mentor students" is from 1887. The meaning "military person sent to help a government or army in a foreign country" is recorded from 1915. Alternative form, Latinate advisor, is perhaps a back-formation from advisory.