虹
コウ, にじ

rainbow

JLPT1 G7 S9 F2110

As per 工# (connecting pipe/tube) + 虫 insect/creature (here, in its original sense of "snake," suggesting a dragon) → vivid *rainbow* connecting the two ends of the sky. Anciently, a rainbow was regarded as one type of dragon.

[Full Kanji Etymology List]