by heart 背得出；靠记忆；单凭记忆；能背诵

【例句】
I've dialled the number so many times I know it by heart.
这个号码我拨了很多次，都记住了。

I had to learn poems by heart at school; I can still recite them now.
我上学那会儿要背诗，到现在我还能背下来呢。