You don't know where it's been

You don't know where it's been.

Do not touch something or put it in your mouth, because you do not know where it has been and what kind of dirt it has picked up. (Most often said to children.) Mother: Don't put that money in your mouth. You don't know where it's been. Bill: Okay. Take that stick out of your mouth. You don't know where it's been.
See also: been, know