unpredictable

When something's unpredictable, it doesn't happen at expected times or it can't be predicted. If trains were unpredictable, they wouldn't follow a set schedule and you'd never know when one would arrive.

The adjective unpredictable can also describe something that's unexpected or not known in advance. Your soccer team's skill level might be wildly unpredictable — from game to game your coach never knows what you guys will do. It can be an insult if someone calls you an unpredictable person. That means people can't rely on you for things, such as consistently good work or keeping your promises.

Definitions of unpredictable
  1. adjective
    unknown in advance
    “an unpredictable (or indeterminable) future”
    Synonyms:
    indeterminable, undeterminable
    not capable of being definitely decided or ascertained
  2. adjective
    not occurring at expected times
    synonyms: irregular
    sporadic
    recurring in scattered and irregular or unpredictable instances
  3. adjective
    not capable of being foretold
    Synonyms:
    uncertain
    not certain to occur; not inevitable
    aleatory
    dependent on chance
    capricious, freakish
    changeable
    episodic, occasional
    occurring or appearing at usually irregular intervals
    unforeseeable
    incapable of being anticipated
    see moresee less
    Antonyms:
    predictable
    capable of being foretold
    certain, sure
    certain to occur; destined or inevitable
    foreseeable
    capable of being anticipated
    inevitable
    invariably occurring or appearing
    show more antonyms...
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