Wiktionary
n. 1 A supposed truth that has been passed down by word of mouth 2 A rumour, myth or superstition; something which is almost certainly untrue, despite acceptance by many.
WordNet
n. a bit of lore passed on by word of mouth
Wikipedia
Old wives' tale is an epithet used to indicate that a supposed truth is actually a superstition or something untrue, to be ridiculed. It can be said sometimes to be a type of urban legend, said to be passed down by older women to a younger generation. Such "tales" are considered superstition, folklore or unverified claims with exaggerated and/or inaccurate details. Old wives' tales often center on women's traditional concerns, such as pregnancy, puberty, social relations, health, herbalism and nutrition.
Usage examples of "old wives' tale".
Gregor Mendel had been dead twelve centuries at that time, yet all the old wives' tale were what most people believed-and still do, I might add.
Something to talk to a pshrink about, perhaps, but as the old wives' tale went, you had to be crazy to go to a pshrink….
An old wives' tale said that if a man died while his lover carried his child, the woman could capture her lover's spirit in the unformed child, so that he would be born again.
He noticed Soarsby had also had garlic for dinner and almost laughed out loud at the thought of that old wives' tale.
Yes, well, I know there are many who believe this to be the actual work of the Builders themselves, but that is an old wives' tale.
Khahkht is an old wives' tale, not to be credited by men of sense.
Gregory knew the later was but an old wives' tale, but in all the years he had known elves, he had never seen one weep.