"内心深处" 的英文表达是 "deep inside"。这个表达的来源可能源于人的内心（心灵）深处所潜藏的思想和情感。

例句：

I know deep inside that I can do this. (我内心深处知道我能做到这一点。)

Despite his tough exterior, he has a deep sense of kindness inside. (尽管他外表强硬，但他内心深处有着深厚的善意。)

Deep inside, she still loves him. (内心深处，她仍然爱着他。)