Possibly an archaic adjective meaning 'shaped like a basket' or 'having a corbel-like form,' though historical usage is unclear.
If derived from 'corbe' or 'corbeil,' would mean 'having the quality of a basket or container,' but the word appears rarely in historical texts and exact meaning is uncertain.
Many -ed adjectives in Middle English were created on the fly by speakers to describe shapes—'corbed' might be someone's one-time description that happened to get written down, making it permanently part of the historical record.
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