In an excellent or first-rate manner; very well or splendidly.
From capital (Latin capitalis, 'of the head') + -ly. The sense of 'excellent' developed because the head was considered the most important part, so 'capital' came to mean 'chief' or 'primary,' and eventually 'excellent.'
Medieval scholars ranked things by importance, with the 'head' always first—that's why capital letters top sentences and why calling something 'capital' meant it was the best. This ranking system gave us a whole vocabulary of superiority!
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