mid-13c., mildeu "honeydew, nectar," from Old English meledeaw "honeydew" (sticky stuff exuded by aphids), from Proto-Germanic compound of *melith "honey" (from PIE root *melit- "honey") + *dawwaz "dew" (see dew). Similar formation in Old Saxon milidou, Dutch meeldauw, German Meltau "mildew."
First element in many cases assimilated to forms of meal (n.2) "ground grain." As a kind of fungus it is first recorded mid-14c., so called from its being sticky and originally growing in plants. As a verb from 1550s. Related: Mildewed.