a made-up story passed off as truth: a fable or legend, used as an insult for teaching the writer wants to discredit
In the New Testament this word is never neutral. It appears five times, and every time it is a put-down. It labels what the gospel is not: invented stories, speculative family-tree legends, fables that people prefer to solid teaching. The writers borrow it from the educated speech of their day, where calling something a fable meant calling it fiction dressed up as fact.
The word had not always carried that sting. In the oldest Greek poetry it simply meant a word, a speech, or an account. But over centuries it soured, until by the first century anyone writing in polished Greek who called a thing a fable was dismissing it as untrue. The Greek translation of the Old Testament had already shied away from the word for its own sacred history, and the New Testament keeps that distance.
The two senses below are shades of one idea, not two different words. Both name a fabricated tale set against the truth (ἀλήθεια). They differ only in what each passage sets the fable against: in most cases it is the false teachers' speculative religious stories, weighed against sound teaching and God's ordered plan; in one case it is a cunningly invented tale, weighed against what the apostles saw with their own eyes.