To malfunction is to fail to work properly. If your toaster spits out a charred, smoky mess no matter what setting it’s on, your toaster is definitely malfunctioning.

When something functions, it works. Adding the prefix -mal (meaning "bad") to function indicates bad or unsuccessful functioning. When a computer malfunctions, you could lose your data. A malfunctioning car could cause an accident or leave you stranded. This word is typically used in reference to technology, tools, and gadgets. However, if you make a stupid mistake, you might joke that your brain has malfunctioned.

Definitions of malfunction
  1. verb
    fail to function or function improperly
    “the coffee maker malfunctioned
    synonyms: misfunction
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    function, go, operate, run, work
    perform as expected when applied
    be operating, running or functioning
    operate in or through
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    get stuck and immobilized
    type of:
    break, break down, conk out, die, fail, give out, give way, go, go bad
    stop operating or functioning
  2. noun
    a failure to function normally
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    a sudden malfunction of a part or apparatus
    a malfunction in the flight of an aircraft in which there is a sudden loss of lift that results in a downward plunge
    a failure to sustain social norms or expected conditions
    type of:
    an event that does not accomplish its intended purpose
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