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.