"surroundings, medium, environment," 1854, from French milieu, "middle, medium, mean," literally "middle place" (12c.), from mi "middle" (from Latin medius, from PIE root *medhyo- "middle") + lieu "place" (see lieu).
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit madhyah, Avestan madiya- "middle," Greek mesos, Latin medius "in the middle, between; from the middle," Gothic midjis, Old English midd "middle," Old Church Slavonic medzu "between," Armenian mej "middle."
late 13c., usually as part of the phrase in lieu of "in the place, room, or stead of," from Old French lieu, lou "place, position, situation, rank" (10c.) from Latin locum (nominative locus) "a place" (see locus).
<a href="https://www.etymonline.com/word/milieu">Etymology of milieu by etymonline</a>
Harper, D. (n.d.). Etymology of milieu. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/milieu