Cain is a name from the Bible: in the Old Testament, Adam and Eve’s first son. His younger brother is Abel. In the story Cain works the ground and Abel keeps animals. God is more pleased with Abel’s gift, and Cain, angry and jealous, kills Abel. He is then marked and sent away. So in English culture Cain is the first murderer, and a picture of a brother who turns on a brother. Always a capital C, because it is a name. It sounds just like cane, the walking stick. Though actually, the Hebrew name is said to have meant “craftsman,” but English speakers almost never think of that when they hear Cain. They think of the brother, and of the anger. One old idiom still walks around: to raise Cain means to make a lot of noise or trouble, as if you were calling that violent brother up. That is a use of the name, not a different word.