劍橋網路辭典有如下的定義與例句
1. If a person, especially a child, acts up, they behave badly:
Sophie got bored and started acting up.
2. If a machine or part of the body acts up, it does not perform as well as it should:
My car always acts up in cold weather.
Her shoulder was acting up (= hurting because of injury).
如何翻譯?
Web Dictionary也舉了許多例句並做翻譯：
Every time it rains our car acts up .
每當下雨，我們的車就失靈。
The children always act up when their mother is out .
母親不在家，孩子們總是調皮搗蛋。
The car acted up because the spark plugs were dirty
汽車出毛病是因為火花塞臟了。
But that was before he started acting up
不過那是在他開始舉止異常之前的事
對不起，「失靈」的英文不是ACT UP，調皮搗蛋、出毛病、舉止異常也不是。 不然，你拿這些中文句子請老美翻譯, 看他們會不會用act up 來表達？
其實 act up 真正的意思是「出狀況」！所以：
蘇菲亞感到無聊，開始出狀況；天氣冷的時候，我的車子就出狀況； 他的肩膀出狀況了；每次下雨我們的車子就出狀況......。
如果你還不相信別人的翻譯是錯的，再舉下面的例子給你看：
act up (idiom) 出狀況。
My youngest child acted up in restaurants often when she was 2 years old. 我最小的女兒兩歲時常在餐廳出狀況。
What if the North Koreans start acting up again? 如果北韓又開始出狀況，那該怎麼辦？
Now, on polluted days, his lungs act up. 現在，在空氣不好的日子，他的肺就出狀況。
He began acting up when he was 3 years old. He would refuse to go to bed and insist in having the television on all night. 他三歲的時候開始出狀況，不願上床，硬是要看整晚電視。
無法區辨英文成語是否翻譯正確是學習英文成語的最大障礙！
