cover one's ass

"cover one's ass"という表現をご存知でしょうか。

米口語の表現なのですが、言い逃れをする、とか隠蔽工作をする、アリバイ作り、というような意味合いで用いられます。

Merriam-Webster Dictionaryでは以下のように定義されています。

to keep oneself/someone else from being blamed or punished

（例文）

I wish he'd admit to his mistakes instead of always trying to cover his ass.

コーパスで用例を検索してみましたが、インフォーマルな口語表現故か、新聞記事の類ではあまり実例を見つけられませんでした。

The book's opening anecdote tells of an unnamed CIA briefer who flew to Bush's Texas ranch during the scary summer of 2001, amid a flurry of reports of a pending al-Qaeda attack, to call the president's attention personally to the now-famous Aug. 6, 2001, memo titled "Bin Ladin Determined to Strike in US." Bush reportedly heard the briefer out and replied: "All right. You've covered your ass, now."

(Houston Chronicle, 2006)

ちなみに、"cover your ass"を略して、単に"CYA"とすることがあるようです。

The newly elected members, knowing the makeup of their electorates and knowing the usual pattern in mid-term elections, would likely be exceedingly skittish from the start. At the same time, the expectations of the left would push the agenda in their direction, creating a very difficult push-pull both for a President Clinton and her speaker ally. If past history is any guide, most of those freshmen will be in full CYA mode, distancing themselves from their party and president.

(The Atlantic, 2016)

"ass"という単語自体、猥雑な響きがあるものですから、特にフォーマルさが求められるコンテクストでは避けられるものと思われます。そういうこともあってか、"ass"に代わって"tail"を用い、"cover one's tail"ということもあるようです。

The inspector, an architectural engineer, "went wild on it," Borchardt said, meaning it wasn't sound. This necessitated bringing in one inspector after another for their opinions. "They all got excessive trying to cover their tails," she said. "And we never really knew what was what."

(Chicago Sun Times, 1994)

以前取り上げた、"ass over teakettle"もご覧ください。