covering one's ass這個美國俚語起源於20世紀60年代越戰期間。當時軍人會提供武力支援「掩護（cover）」給其他同胞，用cover來代替「保護（protect）」這個詞，而ass則是指「屁股」，是比較粗俗不禮貌的說法。
cover one's ass的英語解釋是：To act in order to avoid blame or responsibility for something or prevent oneself from experiencing negative consequences.
看看老外聊天時怎麼用：
Nancy：Mike is in trouble with his wife again.
邁克又惹他老婆生氣了。
Ethan：I know. Mike asked me to cover his ass by lying to his wife.
我知道。邁克要我幫忙騙他老婆掩護他。
Nancy：Did you lie to her？
你有騙她嗎？
Ethan：No，she's a good friend of mine，so I didn't feel right about it.
沒有，她是我的好朋友，所以我覺得這樣不好。
再看幾個例句：
I immediately covered my ass by saying that I wasn't home when the puppy got out.
我立即說小狗跑出去時我不在家，以此來掩飾我的過失。
Politicians don't care about the outcome of the vote, they just care about covering their asses.
政客們不關心投票結果，他們只關心掩蓋事實。
The first thing you learn in the army is to cover your ass.
在軍隊裡你要學的第一件事就是如何保護自己。
Jane is ingenious at finding ways to cover her hide.
簡很善於保護自己。（注意可以用hide代替ass表達同樣含義）