Plausible deniability actually is a legal concept. It refers to lack of evidence proving a allegation. Standards of of proof vary in civil and criminal cases. In civil cases, the standard of proof is "more likely so than not" whereas in a criminal matter, the standard is "beyond a reasonable doubt" If your opponent lacks incontrovertible proof (evidence) of their allegation, you can "plausibly deny" the allegation even though it may be true.
Because many lawyers are in politics, they brought this lower standard of ethics and integrity with them. This is why they rarely put anything controversial in writing. This is also why they most often have you talk to an underling or an agency bureaucrat so they can plausibly deny knowledge of the conversation or be able to say the underling or bureaucrat misstated their position.
Before the blue, stained dress was found, Bill Clinton's position was plausible deniability with regards to having sex with that woman.