One way to hold lunacy: a mind that has come loose from ordinary judgement. Older writers put that under the moon, and used the word for illness and for a court ruling. Modern English keeps the but drops the illness, so today it usually lands on a plan so far from sense that everyone watching shakes their head.
From the word’s history
The packet gives lun(atic) + -acy, and the source's own gloss ties the older sense to the phases of the moon — which is why the historical branches are about illness while today's use is about crazy decisions.
證據裡寫的是 lun(atic) 加 -acy，而來源自己的釋義把舊意思連到月亮的圓缺。這正好說明為什麼歷史上的分支講的是疾病，而今天的用法講的是荒唐的決定。再往前的拉丁字根不在這份證據裡，這裡不作宣稱。
packet evidence: Wiktionary etymology_text for lunacy (From lun(atic) + -acy) and its gloss 'A cyclical mental disease, apparently linked to the lunar phases', source_ref kaikki:3f6f69a190036ae4:425282214:12281
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