空荡荡
kōng dàng dàng · HSK 7
absolutely empty
📦 空 (empty) + 荡荡 (sway-sway) = so empty an echo could swing back and forth
空 — cave radical 穴 + 工 (phonetic)
Originally a phono-semantic compound: 穴 (cave, hole) gives the meaning of emptiness or hollow space, while 工 supplies the sound. The earliest sense was 'hole' or 'cavity,' which extended to 'empty,' 'sky,' and 'in vain.' The reading kòng (4th tone) specifically means 'free time' or 'vacancy.'