A now-debunked story or idea that was once believed, often superstitiously. How can you believe in that old wives' tale?Oh, that's just an old wives' tale! A broken mirror does not guarantee seven years' bad luck.
A superstition, as in Toads cause warts? That's an old wives' tale. This expression was already known in ancient Greece, and a version in English was recorded in 1387. Despite invoking bigoted stereotypes of women and old people, it survives.