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 [kàgéróú] (Heiban – [0])
• IPA^(key): [ka̠ɡe̞ɾo̞ɯ̟]

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.

かげろう • (kagerō) 

1. volitional form of かげる (kageru) [godan]

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