know by heartknow/learn (something) (off) by heart
if you know or learn something, especially a piece of writing, by heart, you know or learn it so that you can remember it perfectly.
He's my favourite poet. I know several of his poems by heart.
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!
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.