My righteousness I hold fast, and will not let it go: my heart shall not reproach me so long as I live. Job 27:6
The New Testament uses the word syneidesis for “conscience”; this English word, derived from the Latin conscientia, “with knowledge” is an accurate equivalent of the Greek–a man “knows with” his inner self if something is not right.
Old Testament Hebrew does not have a word for conscience at all. Modern Hebrew has coined the word matspun from a word which means “to conceal”–we hear the voice of God concealed within us. The same root yields matspen, “compass”–our conscience gives us the direction in our lives.
When the Old Testament speaks of the conscience it uses the word “heart.” Job could say, "My conscience will not reproach me as long as I live" (27:6). The Hebrew phrase lô yeharev levavi, “my heart does not sting me,” underlines the part played by the heart as an indicator of the state of one’s relationship with God.
When David had cut off a corner of Saul’s robe in the cave at En Gedi we read that he was afterwards "conscience stricken" (1 Sam. 24:6). The Hebrew states badly that "his heart beat" inside him. The same phrase is used when the census is taken of the fighting men in Israel and Judah, when David, in his lack of faith, weighs up his own human potential (2 Sam. 24:10)...the act implied a lack of trust in God. Solomon, in 1 Kings 8:38, prays for mercy for Israel since "each one is aware of the afflictions of his own heart.” The phrase nega levavô means ‘heart pains’.
Our hearts ‘sting’ and ‘hurt’ unless we are honest before God.
Santala, Risto. 1992. The Messiah in the New Testament. Jerusalem: Keren Avah Meshihit., pg 144