An archaic or dialectal adjective possibly meaning somewhat average, ordinary, or resembling an average thing.
From avera or average with the diminutive or descriptive suffix -ish. This appears to be a very rare formation, mostly found in Middle English texts, if at all.
The suffix '-ish' is English's great escape hatch—you can add it to basically anything to mean 'kind of like' or 'somewhat.' 'Averish' probably means 'kind of average,' showing how speakers have always been creative in making new words when existing ones don't quite fit.
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