かげろう

Derived as a verb from the noun 陽炎 (kagerō, “heat haze, heat shimmer”).^[1]

The shadow sense may derive instead from verb 陰る (kageru, “to become shady, to become shadowed”), as kagera (the 未然形 mizenkei, “imperfective form”) + ふ (fu, auxiliary verb indicating repeated or continuous action): /kaɡerafu/ → /kaɡerau/ → /kaɡerɔu/ → /kaɡerou/

That said, the kage in kageru is ultimately cognate with the kage in the noun kagerō.

• (Tokyo) かげろう (Heiban – )
• IPA^(key):

かげろう • (kagerou) intransitive ^†yodan

1. (archaic) to shimmer, to flicker
2. (archaic) to become shadowed, to fall into shadow

Only listed with 四段活用 (yodan katsuyō, “quadrigrade conjugation”),^[1][2] one of the conjugation patterns for Classical Japanese, indicating that this fell out of common use before the modern period. Note that the Volitional form in the table below should therefore be かげろわう kagerowau instead.

Reading of various kanji compounds.

かげろう • (kagerō) 

1. 陰郎: during the Edo period, a boy kabuki actor who has not yet appeared on stage; a boy prostitute at banquets
2. 陽炎: heat shimmer, heat haze
3. 蜉蝣, 蜻蛉: a mayfly or dayfly; ephemera, something ephemeral or fleeting