What exactly does the word “clean” even mean in this context? Certainly, that cash isn’t going to get you sick. It also is unlikely to be covered in dirt/mud/s

What exactly does the word “clean” even mean in this context? Certainly, that cash isn’t going to get you sick. It also is unlikely to be covered in dirt/mud/some other substance that will visibly rub off onto you. What ritual must it undergo to purge it of its ungodliness?