I found this on Wiki :

Plausible deniability refers to the denial of blame in loose and informal chains of command where upper rungs quarantine the blame to the lower rungs, and the lower rungs are often inaccessible, meaning confirming responsibility for the action is nearly impossible. In the case that illegal or otherwise disreputable and unpopular activities become public, high-ranking officials may deny any awareness of such act or any connection to the agents used to carry out such acts.