In a sleeping state; asleep or dormant; characterized by sleep or rest.
From 'a-' plus 'slumber,' where 'slumber' comes from Middle Dutch 'slummeren.' This is one of the most poetic of the 'a-' words and was used by English Romantic poets to convey peaceful rest.
Shakespeare and Keats would have loved 'aslumber'—it sounds so much more beautiful than just saying 'asleep'—and it shows how English had a whole arsenal of poetic adverbs we've now abandoned for simpler forms.
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