Relating to or characteristic of a particular ethnic group, nation, or people; of or pertaining to ethnicity or ethnic identity.
From Greek 'ethnos' (people, nation) combined with the suffix '-ical' (relating to), an older form that's sometimes used interchangeably with 'ethnic' today.
While 'ethnical' is an older form of 'ethnic,' you'll rarely hear it in modern English—but it shows how language changes when we gradually drop the extra '-cal' from our adjectives!
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