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Phrase(s): one-track mind
Fig. a mind that thinks entirely or almost entirely about one subject. • Adolescent boys often have one-track minds. All they’re interested in is the opposite sex. • Bob has a one-track mind. He can only talk about football.
McGrawhill's American Idioms And Phrasal Verbs
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A mind limited to only one line of thought or action, as in All you think about is sex? you have a one-track mind. This expression, alluding to a train that runs only on one track or in one direction, was first recorded in 1928.
American Heritage Idioms