かげろう

Etymology 1
Derived as a verb from the noun.

The sense may derive instead from verb, as kagera (the  mizenkei, “imperfective form”) + :  →  →  →

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

Verb

 * 1)  to shimmer, to flicker
 * 2)  to become shadowed, to fall into shadow

Usage notes
Only listed with, 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.

Etymology 2
Reading of various kanji compounds.

Noun

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