A variant plural or Latin-influenced form possibly relating to fondue or a botanical term in scientific naming.
Possibly from Latin 'fondus' or a Latinized plural of French 'fondu.' This appears to be either a rare variant spelling or a technical/scientific term rather than standard English.
This word is mysterious—it might be a Latinized form used in scientific contexts (following the scientific tradition of Latin plurals), or it might be a very rare alternate spelling. Scientific nomenclature loves Latin forms, which is why 'fundus' (meaning 'bottom') exists in anatomical Latin.
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