A foolish, scatterbrained, or frivolously minded person.
Compound word from giddy (frivolous, foolish) plus brain (the organ of thinking), used as a mild insult or descriptor for someone who doesn't think seriously.
Insults reveal what a culture values—calling someone a 'giddybrain' shows that 18th/19th century people really valued serious, focused thinking and mocked those who seemed frivolous.
Complete word intelligence in one call. Free tier — 50 lookups/day.