Completely full, filled to the brim; containing as much as possible.
From 'brim' (the edge of a cup or container) plus 'full.' This has been used in English since the medieval period to describe containers filled to their maximum capacity.
You often see 'brimful' and 'brimfull' both used in texts—the extra 'l' in 'brimfull' comes from people doubling the final consonant before adding '-ful,' which shows how English spelling rules were still being negotiated even in modern times.
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