An older or variant spelling of 'Abbasid,' referring to a member of the Abbasid dynasty that ruled the Islamic world.
From 'Abbas' with the French-influenced '-side' ending (similar to how 'Fatimide' was used instead of 'Fatimid'). This spelling reflects how French and English texts of the 17th-18th centuries rendered Arabic dynasty names.
The '-side' ending was fashionable in older European texts for dynasty names borrowed from other languages—you'll see 'Abbasside' and 'Abbassid' and 'Abbasid' all in historical documents, showing how spelling wasn't standardized until much later.
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