Learn English – How did ‘forthwith’ evolve to mean ‘immediately’

The relevant meaning in French (that is indeed listed in etymonline) is medium.

If you look in the French Larousse from 1863 (via Google books), it says:

Phys. Fluide qui environne les corps: l'air est le milieu dans lequel nous vivons.

This is clearly the meaning adapted into English. Translated, this means

Phys. Fluid that surrounds bodies: air is the milieu in which we live.

Surprisingly, the translation of this physics sense of the French word milieu is the English word medium, which also translates the quite different everyday French meaning of milieu (ie = middle in English). One might think that the French and English physicists had been talking to each other.

Of course, you then get the questions of how the word medium came to have this meaning, and whether medium was used for this in English before milieu in French, or vice versa.

Etymonline says:

Meaning "intermediate agency, channel of communication" is from c.1600.

So my guess is that a substance through which waves are carried is called a medium because it is an intermediate between the sender and the receiver. And this meaning got broadened to be much more general, including (for example) a substance in which bacteria are cultured.

It was believed that epilepsy seizures were triggered by moonlight hence lunatic was used for those patients.

Luna gives the adjective lunaticus. This appears in the Vulgate (405) of the Dalmatian Christian writer Saint Jerome (Eusebius Hieronymus, 348–420) as an epithet for “a moon-struck” person, whence “crazed, insane, lunatic.” It was used of epilepsy, from the notion that the seizures were precipitated by moonlight. The paroxysmal nature of the disease was thought to be dependent upon the phases of the moon.

Lexicon Orthopaedic Etymology

There is also a scientific publication titled The disease of the moon: the linguistic and pathological evolution of the English term "lunatic" from ncbi.nlm.nih.gov that the origin of the word is related to epilepsy:

The original meaning of the term “lunaticus” is not related only to insanity. In particular, its first use is documented in the Vulgate, the fifth-century Latin version of the Bible,translated from the Ancient Greek by Jerome (347–420) on commission of Pope Damasus.

In the Gospel of Matthew (17: 15–18), a father asks Jesus to cure his son because he is “lunaticus” (“Domine, misere filio meo, quia lunaticus est, et male patitur: nam saepecadit in ignem et crebro in aquam. [ . . . ] Et increpavit illum Jesus et exit ab eo daemo-nium et curatus est puer ex illa hora”). This episode is translated in the Bible of King James (1611) as follows: “Lord, have mercy on my son: for he is lunatick, and sore vexed; for ofttimes he falleth into the fire, and oft into the water [ . . . ] And Jesus rebuked the devil; and he departed out of him: and the child was cured from that very hour.” When this passage is compared with the other synoptic gospels (Luke 9: 37–43; Mark 9: 17–29), the most accurate description of the same episode leads us to understand that the boy is affected by epilepsy.

The term “lunaticus est” is the Latin translation of the Greek verb “ σεληνιαζεται ” (“seleniazetai”), which includes the prefix selen- (from σεληνη - the ancient Greek word for the moon). Therefore, the original meaning of the term “lunatic” seems to be linked to epilepsy, rather than insanity.

You can read about the legal category of lunacy and the history of the word in psychiatry in the book The Moon and Madness (By Niall McCrae).