Having footnotes added; marked with or accompanied by explanatory notes placed at the bottom of a page.
From footnote + ed (past participle), where footnote comes from foot (bottom of page) + note (explanatory remark). Footnotes became standard scholarly practice during the Renaissance when printing made it easier to add marginal notes.
A heavily footnoted academic book is like an iceberg—you see the main argument floating on top, but beneath it are hundreds of sources, debates, and rabbit holes that scholars have gone down, essentially documenting the entire conversation around an idea.
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