cover one's ass不是遮住某人的屁股，其实真正意思是指“掩护、掩盖（某人或事）”。

【例句】
I immediately covered my ass by saying that I wasn't home when the puppy got out.
我立即说小狗跑出去时我不在家，以此来掩饰我的过失。

Jack needs you to cover his ass.
杰克需要你帮忙掩盖事实。