An old wives' tale is a type of urban legend, similar to a proverb, which is generally passed down by old wives to a younger generation. Such 'tales' usually consist of superstition, folklore or unverified claims with exaggerated and/or untrue details. Today old wives' tales are still common among children in school playgrounds. Old wives' tales often concern pregnancy, puberty and nutrition.
In this context, the word wife means woman rather than married woman. This usage stems from Old English wif (woman) and is akin to the German weib, also meaning woman. This sense of the word is still used in Modern English in constructions such as midwife and fishwife.
Most old wives' tales are false and are used to discourage unwanted behavior, usually in children, or for folk cures for ailments ranging from a toothache to dysentery. Among the few tales with grains of truth, the veracity is likely coincidental. 
The concept of old wives' tales is ancient. In the 1st Century, the apostle Paul wrote to his young protoge Timothy, "But refuse profane and old wives' fables, and exercise thyself rather unto godliness" (I Timothy 4:7).