The story
The noun is derived from honey + dew, originally believed to be a form of dew that fell from the sky like rain onto plants (see sense 1). The adjective is derived from the noun.
Etymology from Wiktionary, CC BY-SA 4.0.
honeydew
hon·ey·dewToday it means: A sweet, sticky substance deposited on leaves and other plant parts by insects (especially aphids and scale insects) feeding on plant sap, or by fungi. Full definition →