For instance: a very quiet person, who does good deeds all the time and out of nowhere they turn out to be a completely different person. In animal terms, the hippo looks to be innocent, but is actually extremely poisonous and dangerous.

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    Hi @Julia, good question. If you've done any research on this question yourself, please include that information with citations. The site prefers questions that reflect some degree of research on the part of the questioner. Thanks!
    – freeling10
    Commented Feb 3, 2017 at 2:44
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    Pretty sure this is the first time I have ever heard anyone call a hippo poisonous.
    – cobaltduck
    Commented Feb 3, 2017 at 14:27
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    I'm pretty sure hippos are not poisonous, literally or metaphorically. Please do not believe everything that you read, even if it is on modern art.
    – Pharap
    Commented Feb 4, 2017 at 2:27
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    I can see a couple of different interpretations of this question. It would be helpful to know which of them you are looking for. On the one hand, you could be talking about a person who gave the appearance of being virtuous and kind, but this is a front, and they are secretly malicious, but hiding it. Several answers provided already have this meaning. Another possibility is that the person was genuinely good and kind, but something happened which changed their behavior unexpectedly, such as a personal tragedy, insanity, demonic possession, etc. In that case, other phrases might be better.
    – barbecue
    Commented Feb 4, 2017 at 16:46

10 Answers 10


Wolf in sheep's clothing

  • someone or something that seems to be good but is actually not good at all

My grandfather was a wolf in sheep’s clothing – he looked like a sweet old man, but he was really mean.


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    This means they're being deliberately deceptive, though, which hippos aren't. Is that what you're looking for, OP? Commented Feb 3, 2017 at 19:04
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    @MissMonicaE How do we know hippos don't do that purposely? I've personally never interviewed one.
    – Hank
    Commented Feb 3, 2017 at 19:05
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    Okay, fair point. Maybe they are! Commented Feb 3, 2017 at 19:13


  1. Having no adverse effect; harmless.


Depending on your sentence, innocuous looking might work better.


Thanks to @AndyT: or seemingly innocuous.

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    Or seemingly innocuous.
    – AndyT
    Commented Feb 3, 2017 at 10:05

Butter wouldn't melt in her mouth:

Be overly coy or demure; be insincere. For example, She looked quite innocent, as though butter wouldn't melt in her mouth, but we knew better. Already a proverb in John Heywood's collection of 1546, this metaphoric expression alleges that one is literally so cool that butter inside the mouth would not melt.


Used in my experience of someone who is actively attempting to appear innocent whilst not being so. Often used sarcastically where the attempted deception is transparent, but also as a warning where the deceit has apparently been successful.

  • Good answer Magoo, exactly what I was thinking. I've formatted it to make it clear what is quoted from your link (which I've turned into a hyperlink) and what is your own contribution/opinion (which I 100% agree with).
    – AndyT
    Commented Feb 3, 2017 at 10:02
  • This would have been my answer too, good one! Commented Feb 3, 2017 at 21:43
  • A personal favourite phrase of mine, I often use it when discussing my pet cat's behaviour. (p.s. I need a new set of headphones... again...)
    – Pharap
    Commented Feb 4, 2017 at 2:18

They are perhaps disingenuous.



not really honest or sincere, and only pretending to be



  • Dangerous, especially when seeming safe (Oxford)
  • Guilty of or characterized by betrayal of confidence or trust; perfidious. (TheFreeDictionary)

You could also use snake in the grass


On a somewhat different tack than other answerers, what about the word Machiavellian?

Most dictionary definitions just say "someone adhering to the principles of Machiavelli" and I think that's because it's just difficult to define precisely what is considered "Machiavellian." On dictionary.com:

  1. characterized by subtle or unscrupulous cunning, deception, expediency, or dishonesty:

I would define it more as someone who is ruthlessly pragmatic and without conscience. Such people may appear guileless and helpful at first, until or unless they discover a way to use you or the situation for their own profit. The word is often used to describe psychopathy/sociopathy, because psychopaths so often are superficially charming, but only behave that way because it gets them what they want.


The adjective insidious is close to your intended meaning:

  1. intended to entrap or beguile: an insidious plan.
  2. stealthily treacherous or deceitful: an insidious enemy.
  3. operating or proceeding in an inconspicuous or seemingly harmless way but actually with grave effect: an insidious disease.



There is a a phrase from popular culture that fits this meaning, but it's not literary in origin, rather it comes from the low-brow realm of professional wrestling.

When a virtuous or good person suddenly and surprisingly turns into a villain, they are said to turn heel. Note that this is a different meaning of "turn heel" from the older phrase meaning to turn around, as in to turn heel and run.

This phrase originates in the world of professional wrestling, where wrestlers are generally divided into heroes, called "Faces" (from babyface) and "Heels" (from the early 20th century slang term for a disreputable person) being the villains. The sudden switch from face to heel is a way to generate drama and excitement.

Professional wrestlers turning heel may not be highbrow entertainment, but it's definitely big business, big enough to earn mention in Forbes magazine, for example.

The popularity of this entertainment has led to the terminology being used in other areas, such as politics and The same tropes also occur in daytime TV dramas (soap operas.) Writers will use this device to help stir interest in a story, but I can't find any specific phrases with this meaning from that genre.


Depending on the exact context, that person might be considered a subversive, which is typically defined as:

(n.) a subversive person.

synonyms: troublemaker, dissident, agitator, revolutionary, revolutionist, insurgent, insurrectionist, insurrectionary, renegade, rebel, mutineer, traitor "she was designated as a dangerous subversive"

As you can see, these synonyms all broadly cover the same meaning - one who is hiding amongst the crowd, pretending to be the same as the majority, while secretly working to undermine the status quo.

To directly answer your example, a subversive may do good deeds all the time as part of their disguise, but out of nowhere they turn out to be a completely different person when they see an opportunity to realise their hidden agenda.

  • I don't mind being wrong, but could somebody please explain the reason for all the downvotes?
    – flith
    Commented Feb 8, 2017 at 11:04

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