1. [for something unpleasant] to appear or become obvious after lying hidden.(freedictionary)

보통 이 표현은 ‘<noun> rear its ugly head’ 형태로 쓰이고

보통 <noun>에 들어가는 명사는 기분을 안좋게하는(이게 포인트입니다) 무언가 입니다.

• Meaningless coincidence might have reared its ugly head after all.
• Olympic doping rears its ugly head again in Athens.
• racism had reared its ugly head again .
• Cyber-Bullying has reared its ugly head
• Jealousy reared its ugly head and destroyed their marriage.
• is inflation rearing its ugly head?

이 표현이 재미있는 이유는 시각적이기 때문인데요, 기분 안좋은게 마치 못생긴 두더지처럼 서서히 머리를 내민다고 생각해보세요. 생각만해도 짜.증.나지 않나요?

저는 또 이렇게도 생각합니다. 예전에 영화 링 기억하시나요? 우물에서 여인이 머리부터 나오는 장면말입니다(싫어하시는 분도 계실까봐 사진은 첨부하지 않았습니다). 되게 기분 더럽고 무섭게 등장하죠? 그런 악몽같이 안 좋은게 서서히 나타난다고 생각해보세요. 정말 끔찍하겠죠?

• He helped me. …as this is not just the macabre conjectures of a psychic, but a foregone conclusion just waiting to rear its ugly head. Even though police would …(Saw VI )
• Scarcely the moment for crime to rear its ugly head. Press on, merry marksmen. We’ve much to do ere night descends. Hyah! I’ve run it up the flagpole back at …(Batman)
• human reality will eventually rear its ugly head. Far more horrific than any film can attempt to portray. I can see her. She sleeps so calmly. Her bloated midsection slowly rising …(Subconscious Cruelty|)

• Apr 26, 2017 - … over “Net Neutrality” is set to rear its ugly head again soon and why Congress needs to create a federal law lining up the principles regulating … (Bloomberg Markets: Downes on Return of Net Neutrality Fight)
• Jan 14, 2011 - Related to “like me” bias, stereotyping can rear its ugly head in the context of appraisals. It can qualify as a form of unlawful discrimination. (The Dirty Dozen Performance Appraisal Errors - Bloomberg)
• Feb 8, 2017 - It’s very difficult to predict when volatility will rear its ugly head, but it will happen at some point in the future. Investors need to understand that … (The Becalming of Stock Volatility - Bloomberg)
• I think we’re seeing an old problem rear its ugly head. Previous incarnations have looked like this: Spending is out of control, it should be cut … but not for … (Comments on How moderate is too moderate? | The Economist)
• Jul 14, 1985 - In these films, the notions of closeness and happiness are so artificial that actual failure need not even rear its ugly head. Alongside the … (FILM VIEW - TRACING THE LONG SHADOW CAST BY 'THE BIG ...)

• reared its bloody head
• AIDS is coming back. Ebola seems to be rearing its head
• when an old disease rears its head, like anthrax.

• Is it a slang? no.
• Can I use it in a formal setting? no.

1. rear its ugly head 에서 ugly 를 생략하거나 또는 더 강하게 안좋은 단어를 써도 괜찮습니다.
• rear its head (O)
• rear its bloody head (O)
• rear its fucking head (O)