One of the two Hebrew words for fool is the word evil, which is spelled just like our word "evil" but is pronounced ev-eel'. The root for this word means weak minded, but it also carries the idea of being strong willed and determined.
The other Hebrew word for fool is chesil, pronounced kes-eel'. It's root meaning is fat. It implies one who is mentally fat, not in the sense that he if full of intelligence but that he is dull, inert, and sluggish. This person's character, not necessarily his outward appearance, is a big blob. He is a lazy, spoiled, and inactive fool.
The Bull-headed Fool & Sin This person mocks at sin. He thinks sin is funny. He doesn't fear it, but instead like it. In fact, he enjoys the righteous shocked by his own sin.