Here is a definition of circumcision from thefreedictionary.com
To remove the prepuce (foreskin) of (a male). [Middle English circumcisen, from Latin circumcdere, circumcs-, to cut around: circum-, circum- +caedere, to cut; see ka-id- in Indo-European roots.]
Here are some passages which refer to the circumcision of the heart:
Deuteronomy 30:6; Leviticus 26:41; Jeremiah 4:4,14; Romans 2:28-29; Colossians 2:11
The context of these passages indicates that, just as the act of circumcision in the Old Testament, and that of Baptism in the New Testament, is an overt act of obedience signifying one's entry into covenant relationship with God, the ongoing "cutting off of the flesh" is a process of obedience and submission to the will of God. We can only determine what that will is by paying close attention to God's word, the Bible.
Joshua 1:7,8 Only be strong and very courageous, that you may observe to do according to all the law which Moses My servant commanded you; do not turn from it to the right hand or to the left, that you may prosper wherever you go. 8 This Book of the Law shall not depart from your mouth, but you shall meditate in it day and night, that you may observe to do according to all that is written in it. For then you will make your way prosperous, and then you will have good success.
Psalm 119:9 How can a young man cleanse his way? By taking heed according to Your word.
2 Timothy 3:15-17 and that from childhood you have known the Holy Scriptures, which are able to make you wise for salvation through faith which is in Christ Jesus. 16 All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, 17 that the man of God may be complete, thoroughly equipped for every good work.
James 1:21-25 So get rid of all the filth and evil in your lives, and humbly accept the word God has planted in your hearts, for it has the power to save your souls. 22 But don't just listen to God's word. You must do what it says. Otherwise, you are only fooling yourselves. 23 For if you listen to the word and don't obey, it is like glancing at your face in a mirror. 24 You see yourself, walk away, and forget what you look like. 25 But if you look carefully into the perfect law that sets you free, and if you do what it says and don't forget what you heard, then God will bless you for doing it.
I hope you have found this helpful.