As a non-lawyer, I often see the phrase "friend of the court".
It is obvious to the layman what it means, but when and where did this first become "a thing"?
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The amicus curiae1 probably has its roots in English Common Law and not, as Wikipedia says, in Roman Law.
Notwithstanding, it’s very old: possibly pre-dating the Norman invasion. As such, it’s origin and early development would be a very niche area of study for a historian and is likely to have fragmentary records.
1 Crema, Luigi. "The Common Law (And Not Roman) Origins of Amicus Curiae in International Law – Debunking a Fake News Item" Global Jurist, vol. 20, no. 1, 2020, pp. 20190038. https://doi.org/10.1515/gj-2019-0038