In bed; lying down in bed, usually for sleep or because of illness.
From Old English 'a' (in, at) plus 'bed.' This is an archaic or poetic form that was common in older English literature but is rarely used in modern everyday speech.
This is a beautiful relic of older English—we've mostly replaced 'abed' with simpler phrases like 'in bed,' but you still find it in Shakespeare and poetry. It's one of those words that makes language sound more lyrical and old-fashioned.
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