For this people's heart is waxed gross, Matthew 13:15.
At two different times our King James Bible describes the heart of the Jewish people in time of Jesus Christ and in the Book of Acts as having waxed gross. Each time that it is used, once by Jesus Christ and once by the Apostle Paul, they make it plain that they are making reference to Isaiah 6:10. Make the heart of this people fat, and make their ears heavy, and shut their eyes; lest they see with their eyes, and hear with their ears, and understand with their heart, and convert, and be healed.
Our bible is indeed colorful and descriptive. Waxed gross, what a perfect way to describe the collective heart of the Jewish people in the first century AD! Isaiah called their heart fat. In his lectures on the Book of Leviticus (Click Here), Dr. Dale Morey has explained the virtues of fat. Fat was precious. It added flavor to meat. It was an accelerant for the fire. But fat in the wrong places is gross. They are inclosed in their own fat: with their mouth they speak proudly, Psalm 17:10. Their heart is as fat as grease; but I delight in thy law, Psalm 119:70.
As usual, the new versions can't get it right. The ESV says; For this people's heart has grown dull. The NIV likewise misses the mark; For this people’s heart has become calloused. Any child can understand God's thinking when he sees a heart that has become fat. "Eww, gross!" Lest anyone think that I exaggerate, look at the Oxford English Dictionary's definition for gross in the this context; Overfed, bloated with excess, unwholesomely or repulsively fat or corpulent.
Keep in mind, there is a collective heart of a people. Isaiah 6:9 is not describing the heart of individual Jews. He is describing the collective heart of the Jews in the time of Christ. The destruction of that people in the first century is a grief to all who love the Jewish people. To love God is to love those people. To hate those people is hate God. We look for that day when the collective heart of Israel can turn to their God. But even unto this day, when Moses is read, the vail is upon their heart. Nevertheless when it shall turn to the Lord, the vail shall be taken away, 2nd Corinthians 3:15,16.