航空
こうくう · koukuu · JLPT N3 · noun
aviation
✈️ 航空 = navigate (航) + sky (空): sailing the sky
航 means to sail or navigate, as in 航海 (ocean voyage); 空 is sky, as in 空港 (airport). Ships navigate seas; aircraft navigate the sky.
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.'