空儿
kòngr · HSK 3
time
⏳ 空 (empty) + 儿 (-r suffix) = empty slot of time = spare time
空 means empty or vacant; the 儿 suffix is the northern colloquial ending. An empty slot in your schedule is free time. Compare 有空 (have free time) and 没空 (no free time).
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.'