A plant species or a cup-shaped flower with a gilded or golden appearance, or a decorative gilt cup.
From 'gilt' (covered with gold or golden-colored) + 'cup' (from Latin 'cuppa'). Either a descriptive name for a flower or a piece of decorative metalwork.
Common flowers and objects got compound names by describing their appearance—giltcup's name tells us exactly what a medieval person saw: a cup-shaped flower that looked gold, no scientific nomenclature needed.
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