I have a hypothetical question: Say I am a normal person, I go on the internet, and buy a prepaid debit card, that seems completely normal and legitimate. The very next day, the FBI knock down my door and rush in and tell me that the debit card I bought was laundered money. Do I have plausible deniability? Like, will I go to jail for money laundering, if I never knew that the card was loaded with laundered money?
Federal money laundering statutes have intent and/or knowledge as an elements of the crime. (DOJ Money Laundering Overview)
E.g. 18 U.S.C. § 1956(a)(1):
with the intent to promote the carrying on of specified unlawful activity
or, 18 U.S.C. § 1956(b)(1):
knowing that the transaction is designed in whole or in part to conceal or disguise the nature, the location, the source, the ownership, or the control of the proceeds of specified unlawful activity
If the prosecution can prove intent or knowledge, you may be found guilty. If you had no intent or knowledge, this will be difficult for the prosecution to do. "Plausible deniability" is simply the prosecution failing to provide evidence that establishes the elements of a crime beyond a reasonable doubt.