Milew refers to certain kinds of mold or fungus. In Old English, it meant honeydew (a substance secreted by aphids on leaves, formerly thought to distill from the air like dew), and later came to mean mildew in the modern senses.
Trivia about mildew
Fungicide may be needed when the "powdery" type of this appears in late summer
The two main types of this fungus that attacks plants are powdery & downy