A type of berry or fruit, possibly one that produces giddiness or dizziness when consumed.
From giddy plus berry, likely a folk or colloquial name for a plant that causes dizziness or intoxicating effects when eaten.
Folk plant names often describe the effect the plant has—a 'giddyberry' would be something that makes you dizzy when you eat it, which is how people remembered which wild plants were safe or dangerous.
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