Second Timothy 2:15, NKJV
"Be diligent to present yourself approved to God, a worker who does not need to be ashamed, rightly dividing the word of truth."
Part 1: Origin of the word "circumcision"
The word "circumcision" derives from old Latin words: "circum" meaning around and "caedere" meaning to cut off. This cutting off of the foreskin is not meant to be cruel, but rather something surgical and intentional.
Part 2: Context of the first circumcision
Genesis 17 is when the first circumcision occurs. Context is key when studying and interpreting Scripture, so as to avoid misinterpretation. The first circumcision took place in the context of God talking to Abraham, the first man after Noah, who was promised to be "the father of many nations" (see Genesis 12:1-3). In order to show that whoever was born is set apart for God, they had to be circumcised. This was part of the dedication process. Any male children who were eight days old were circumcised; otherwise they broke the covenant God made for Abraham and the people of Israel after him.
Part 3: The why behind circumcision
Circumcision was (and still is) the physical representation of a man being sanctified by the Lord to do His will.
