Cover Someone's Ass Meaning

(idiomatic) To make preparations or take precautions to ensure that a person is not blamed or punished for his or her conduct.

Example: 2004, Jim Drinkard and Dave Moniz, "Secretary: Memos are forgeries," USA Today, 14 Sep.,
  Killian kept the files because "he was trying to cover his ass," Via said. "He was always worried something would come back on him."