架空
かくう · kakuu · JLPT N2 · na-adj
imaginary
🦄 架空 = build (架) + empty sky (空): built on nothing, so made-up
架 means to build or put up, and 空 means sky or empty. Something built into empty air is not real, so the word means fictional or fanciful.
架 — wood 木 + add 加 (phonetic)
A semantic-phonetic compound: 木 (wood) gives the meaning, since a 架 is a wooden frame, rack, or shelf used to support things. 加 (jiā) supplies the sound. From 'wooden frame' it extended to verbs like 'to set up,' 'to prop up,' and to a measure word for framed objects such as airplanes.
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.'