A slang term used to describe someone or more than one person (can be as large as a business or any other entity) who is extremely unreliable, unless they want something from you. To put these dirtbags in their place, one must confront them directly face to face. This term is different than a flake, as a fly by night DOES contact a person, but only when they want something, whereas a flake or flaky person DOES NOT.
Tim was such a fly by night about turning in that homework for our group, he got us an F! But then when he wanted a ride home from the bar, he contacted us left and right! That guy can walk his ass 3 miles to get home!
Kurt's fly by night attitude caused him to not be liked by his friends, and for him to only get sex from more undesirable types of women with hairy vaginas.