Hardness (4457) (porosis from poroo = to harden, petrify, render insensitive) literally describes the covering of a part with a callus or thick growth of skin (Hippocrates used it as a technical medical term). Have you ever done hard manual labor especially the type that required you to repetitively use your hands? If you have, you probably developed a callus and you can better understand what Paul is portraying with this word. In this verse he is using porosis figuratively to describe the moral or ethical hardness, callousness, blindness or insensitivity of the hearts of the Gentiles in their former unregenerate state. Porosis expresses the condition of moral insensibility "the deadness that supervenes when the heart has ceased to be sensible of the stimuli of the conscience" (Ellicott). There are 3 uses of porosis in Scripture and here are the other two uses... Mark 3:5 And after looking around at them with anger, grieved at their hardness of heart, He said to the man, "Stretch out your hand." And he stretched it out, and his hand was restored. Romans 11:25 (note) For I do not want you, brethren, to be uninformed of this mystery, lest you be wise in your own estimation, that a partial hardening has happened to Israel until the fulness of the Gentiles has come in; Barclay writes that... Porosis comes from poros, which originally meant a stone that was harder than marble. It came to have certain medical uses. It was used for the chalk stone which can form in the joints and completely paralyze action. It was used of the callus that forms where a bone has been broken and re-set, a callus which is harder than the bone itself. Finally the word came to mean the loss of all power of sensation; it described something which had become so hardened, so petrified that it had no power to feel at all. That is what Paul says the heathen life is like (Eph 4:18) It has become so hardened that it has lost the power of feeling. In the Epistle to a Young Friend, Robert Burns wrote about sin: “I waive the quantum o’ the sin, The hazard of concealing; But och! it hardens a’ within, And petrifies the feeling!” The terror of sin is its petrifying effect. The process of sin is quite discernible. No man becomes a great sinner all at once. At first he regards sin with horror. When he sins, there enters into his heart remorse and regret. But if he continues to sin there comes a time when he loses all sensation and can do the most shameful things without any feeling at all. His conscience is petrified. (Ed note: This is because all men in Adam are totally depraved and have an inherent sin nature from Adam to commit sins). (Barclay, William: New Testament Words:. Westminster John Know Press, 1964)