The quality of being dreamy or dreamlike; a state of drowsy abstraction.
From dreamy + -ness. Dreamy itself comes from dream + -y (suffix meaning 'having the quality of'), so dreamsiness is a doubly-derived noun expressing the abstract quality of dreaminess.
There's actually a subtle difference between 'dreaminess' and 'dreamsiness'—if used at all, 'dreamsiness' might suggest a heavier, more pronounced drowsiness, though 'dreaminess' is the standard modern form.
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