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What is an adjective that describes a person who is unpredictable in a negative or potentially dangerous way? I keep thinking "volatile," but I'm looking for a word more cohesive with the description of a person rather than a science experiment or something. Perhaps something similar to "loose cannon," but an adjective? I believe I am looking for a specific word, but I can't think of it.

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Ditzy, though the connotation is not strongly negative. – Hot Licks Feb 6 at 1:29
    
How about loose cannon? – Nasser Feb 6 at 4:42
    
@Nasser I have that in the question; I'm looking for an adjective. – PlasmaStarfish Feb 6 at 4:49

Volatile is a very good word, not at all necessarily scientific, but connotes explosive.

  • liable to change rapidly and unpredictably, especially for the worse.
  • (of persons) disposed to caprice or inconstancy; fickle; mercurial (Collins/TFD)

A clinical word for this is labile:

Open to change; readily changeable or unstable (TFD)

For example,

Emotional Dysregulation is also referred to as labile mood (a marked fluctuation of mood) or mood swings.

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I think mercurial is actually close... for some reason I just think there is another word better than volatile. – PlasmaStarfish Feb 6 at 2:23
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"Labile" is excellent. We could probably find a bushel of equivalents in the DSM-V, e.g., "bipolar." – Rob_Ster Feb 6 at 2:28

Unstable

Erratic

Impulsive

Fickle

In order of how negative they are in my opinion. There are a few other less ideal ones, like "iffy" and "capricious", but those are the four I'd consider the most negative/dangerous sounding synonyms for unpredictable in terms of people.

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I rather like "capricious" here. It feels quite dangerous to me. – Rob_Ster Feb 6 at 2:31

Many good words have been suggested. You could also consider using the adjective mercurial which means:

Subject to sudden or unpredictable changes of mood or mind: 'his mercurial temperament'.

More broadly used one is capricious:

Given to sudden and unaccountable changes of mood or behaviour

[Oxford Online Dictionary]

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I can't top volatile (my favorite answer here) or unstable, but another possibility is sociopathic, though it may be too "clinical" for your purpose. Psychotic might work, too, though it isn't an exact synonym of the other words suggested here.

P.S. Thanks for adding labile to my vocabulary!

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Several synonyms have been suggested; considering that you specified:

I believe I am looking for a specific word, but I can't think of it.

I thought I would point out a few words that are more similar in sound to "volatile" than the excellent words so far suggested, and that are at least somewhat similar in meaning as well (or at least related). Hopefully one of these may jog your memory:

  • vacillating
  • vindictive
  • virulent
  • vitriolic

And, some more common words I found during my search that I think are unlikely to be the word you are looking for, but might be:

  • variable
  • vicious
  • violent
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Adding to the above list:

Whimsical

Wayward

Impulsive

Fluky

Dicey

Waggish

Wiggly

Wobbly

Arbitrary

Chimerical

Quicksilver

Fickle.

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