Singing a descant; performing an ornamental melodic line above the main tune; also discussing something at length or elaborating on a topic.
Present participle of 'descant'. The musical usage dates to medieval times, while the discursive meaning evolved during the 16th-17th centuries as metaphorical language development.
When you're 'descanting' about something, you're literally 'singing above' the main topic—the metaphor of adding an ornamental melody transferred to how language layers ideas on top of each other.
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