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think twice (before doing something)
to consider carefully whether one should do something; to be cautious about doing something. You should think twice before quitting your job. That's a serious decision, and you should certainly think twice.
think twice (about something)
to consider something more carefully You may want to think twice before buying one of California's new earthquake insurance policies.
Usage notes: often used as a warning, and sometimes used in the form think twice before doing something: He said he is sorry and will think twice before giving such advice again.
1. Reconsider something, weigh something carefully, as in I've got to think twice before spending that much on a car. [Late 1800s]
2. not think twice. Take no notice, not worry about, as in She didn't think twice about flying off to Europe with a day's notice. [Mid-1900s]
To weigh something carefully: I'd think twice before spending all that money on clothes.