Luck is chance or happenstance. If something happens from pure good luck, it seemingly came out of nowhere, based only on fate and not on anything you did to make it happen.

If you find a fifty dollar bill on the sidewalk, you might exclaim, "What luck!" Alternately, if you lose all night at the poker table, you'll probably blame it on bad luck. In both examples, you're referring to a mysterious, unpredictable circumstance that affects your life by chance. Luck has a Middle Dutch root, luc, which is a shortened form of gheluc, "happiness or good fortune."

Definitions of luck
  1. noun
    an unknown and unpredictable phenomenon that causes an event to result one way rather than another
    “bad luck caused his downfall”
    synonyms: chance, fortune, hazard
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    bad luck, mischance, mishap
    an unpredictable outcome that is unfortunate
    even chance, toss-up, tossup
    an unpredictable phenomenon
    type of:
    any state or process known through the senses rather than by intuition or reasoning
  2. noun
    an unknown and unpredictable phenomenon that leads to a favorable outcome
    “it was my good luck to be there”
    “they say luck is a lady”
    synonyms: fortune
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    fluke, good fortune, good luck
    a stroke of luck
    good luck in making unexpected and fortunate discoveries
    type of:
    any state or process known through the senses rather than by intuition or reasoning
  3. noun
    your overall circumstances or condition in life (including everything that happens to you)
    “the luck of the Irish”
    synonyms: circumstances, destiny, fate, fortune, lot, portion
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    good fortune, good luck, luckiness
    an auspicious state resulting from favorable outcomes
    a manifestation of God's foresightful care for his creatures
    bad luck, ill luck, misfortune, tough luck
    an unfortunate state resulting from unfavorable outcomes
    lack of success
    prosperity, successfulness
    the condition of prospering; having good fortune
    blessing, boon
    a desirable state
    the condition of being financially weak
    adversity, hard knocks, hardship
    a state of misfortune or affliction
    gutter, sewer, toilet
    misfortune resulting in lost effort or money
    hard cheese
    bad luck
    a state of complete lack of some abstract property
    bank failure
    the inability of a bank to meet its credit obligations
    crop failure
    the failure of crops to produce a marketable surplus
    dead duck
    something doomed to failure
    type of:
    a mode of being or form of existence of a person or thing
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