Given a familiar or shortened name; called by an informal alternative name instead of a real one.
From 'eke-name' (an additional name), where 'eke' means 'also' or 'extra.' The 'n' from 'an eke-name' was reinterpreted as part of 'nickname' in the 15th century.
This is a perfect example of rebracketing—'an ekename' got misheard as 'a nickname' because people heard the 'n' as belonging to the article instead of the word! The same thing happened with 'napron' becoming 'apron.'
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