An archaic or dialectal form meaning 'under', 'below', or 'beneath', sometimes used in poetic or Old English contexts.
From Old English 'onunder', combining 'on' (at) with 'under', reflecting a earlier stage of English prepositions before they simplified.
English used to have way more prepositions strung together—anunder shows how our language gradually dropped the extra 'on' and simplified to just 'under', which happened as English speakers got lazy in the best possible way.
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