something extremely light; trifle
Figurative use emphasizing extreme lightness or insignificance. Often appears in the set phrase 鴻毛の軽き (こうもうのかろき) meaning 'as light as a feather'.
feather of a large bird
Original literal meaning; now rare outside classical or explanatory contexts.
His life was regarded as light as a feather.
Literally, 鴻毛 refers to the feather of a large bird.
From Chinese 鴻毛 (hóngmáo), literally 'feather of a swan goose', used figuratively for something very light or insignificant.