The answer to your question is actually "un cuento chino". There have been instances where the entire idiomatic expression "cuento de viejas" has been used. However, upon performing more research you will find that "cuento chino" embodies the exact meaning of old wives' tale. A very popular phrase is:
iDejemonos de cuentos chinos!
Which means: Let us quit listening/telling old wives' tales! The strength of the expression lies on the word "cuento". In Spanish, when the word "cuento" is used to purposely replace the word "historia", or "relato", the intended meaning changes completely. Hence, "cuento" (tale), is used to denigrate or downplay the origin of the information that is being related.
Why "chino"? You may ask. There are two explanations that are often given, but there is no official answer. One of the reasons is that in Hispanic cultures there is a deeply engrained respect for the Eastern culture, their history, and their use of natural cures and home remedies. However, fake home cures and remedies proposed by "other than Eastern" curanderos were also disguised as "Chinese" (generalized, not intended as a racial label) remedies to gain instant validity. The story goes that, apparently, plenty of fake cures and remedies have been found out to the point of coining fake tales as "cuentos chinos".
Another background for the term "cuento chino" which makes more sense are the famous short, Asian legends which are told over and over as folklore and are fantastic and mythical. The fact that they are myths and fantasies make them of course, fake. Hence, whenever someone tells an elaborately fake story, the instant connection is to relate it to one of those legendary tales from long ago.