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In ancient thought, at least with respect to the Hebrews, the "heart" (לב) was considered to be the locus of one's thoughts. Today, most might say that thoughts originate from the brain and/ or mind.
Nevertheless, the idea of "to think to one's self" is expressed by the phrase לאמור בלב, which means, "to say in the heart" (cp. Gen. 17:17). This proves the connection they gave between "thought" and "heart."
To circumcise the foreskin of one's heart is to remove all the excess and unnecessary thoughts from one's thoughts and life and to devote one's entire attention and self to serving God. The foreskin signified pleasure and excess.
Were there any specific verses you had in mind?
Wilhelm Gesenius covered the topic well in his entry on the word לב in his lexicon, pp. 507-508.