An old wives’ tale
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
Used informally, great for casual conversations but not in formal writing. 
My grandma warned me that sitting too close to the TV is bad for my eyes, but that's probably an old wives' tale.
Pro Tip 2/3Non-scientific Claims
Refers to beliefs that are not backed by modern science, often passed down through generations. 
I heard that rubbing a wart with a potato will make it disappear, but it sounds like an old wives' tale to me.
Pro Tip 3/3Skepticism Expressed
Express skepticism or doubt about the truth of something when using this phrase. 
Eat chicken soup to cure a cold? Sounds like an old wives' tale to me.