What the sentence means (according to US usage) is that these two rules were forcibly, by repetition, imprinted into the brains of people when they were in school. " This is for convenience and brevity, and this also enhances the metaphor presented here of an electric drill boring a hole into a surface.
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AnonymousWhat the sentence means (according to US usage) is that these two rules were forcibly, by repetition, imprinted into the brains of people when they were in school.A more complete version of the sentence might be: "Most people...probably drilled into (not "in") them at school." This is shortened to: "Most...drilled in at school." This is for convenience and brevit