A lunatic is someone who was driven insane by the evil eye. Moonstruck means affected in a negative way by beams/rays of energy from the eyes. Evidence for that hypothesis is as follows. First the word moon is a metaphor for eyes. Secondly the evil eye was said to cause madness. Evidence below.
The evil eye was said to cause madness:
"Renaissance philosophers such as Marsilio Ficino said that sorcerers (venefici) gave people fevers and caused madness by sending out vapors from the eyes which infected the blood."
https://ultraculture.org/blog/2015/02/27/5-ways-to-protect-yourself-from-the-evil-eye/
http://archive.is/uCKXC
In Norse poetry the eyes are called moons:
https://skaldic.abdn.ac.uk/db.php?if=default&table=kenning&val=EYE
https://skaldic.abdn.ac.uk/db.php?if=default&table=kenning&val=EYES
The fact that eyes are compared to moons and that the evil eye is said to cause madness leads me to suspect that calling madness "lunacy" or being "moonstruck" is a poetic way of saying madness is caused by the evil eye: the hatred and jealousy others feel that is directed towards someone can cause them a type of brain damage that can lead them to behave in a irrational way, it can cause madness.
In ancient Greece they believed that rays of light extend outward from the eyes towards what was looked at. This is evident in Timaeus.
https://librivox.org/timaeus-by-plato/
Evidence West Europeans may agree with this is "Heads sun" is an anglo saxon kenning for eye.
http://bosworth.ff.cuni.cz/018245
Some Greek ideas on Evil eye
http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0137%3Abook%3D7%3Achapter%3D2#note-link24
Above is pliny the elder book vii chapter 2 of natural history