You old fly-by-night; an ancient term of reproach to an old woman, signifying that she was a witch, and alluding to the nocturnal excursions attributed to witches, who were supposed to fly abroad to their meetings, mounted on brooms.
FLY-BY-NIGHT
Meaning of FLY-BY-NIGHT in English
Slang English vocab. Английский сленговый словарь. 2012