know (something) by heart

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know (something) by heart

To have learned or memorized something completely. Ask Becky to recite the poem—she knows it by heart. I can't sing that song in front of an audience if I don't know the words by heart!
See also: heart, know

know something by heart

Fig. to know something perfectly; to have memorized something perfectly. I know my speech by heart. I went over and over it until I knew it by heart.
See also: heart, know