There are a number of words Jesus uses in a very different way to how we use them. He literally redefines and teaches us a new meaning of these words. This is because we've taught ourselves incorrectly what something IS, and have mis-identified things. In ACIM His use of terms gives them a whole different meaning.
Life. You think it means bodies that "are alive". He means, that which has eternal life in Heaven and extends forever, which never perishes and cannot die.
Death. You think it means death of the body. He means, anything even slightly less than 100% perfect life. Even a slight grievance. Even a slight sickness. The entire dream world and everything in it.
Murder. You think it means some person in the extreme attacking another. It really means the dynamic of a separation between cause and effect in which you perceive something applies to you that doesn't apply to someone else, allowing you to escape the consequences of what you cause. The entire ego thought system is murderous. "What is not love (100% love) is murder."
Insanity. Any state of mind which is not total wholeness and holiness. A split mind. A mind which believes illusions are real. A mind which sees itself as separated from its creator. All states of dreaming. Happy dreams, unhappy dreams. All states of perception. Seeing yourself as a body. Believing in death.
Sickness. You think it means just body sickness like having a disease. Jesus means a sick mind which produces a sick body, all states of mental unhealth, death, forms of physical sickness, illness, fear, unforgiveness, guilt, body-identification, belief in sin, level confusion, magic, etc.
Fear. You think it's some feeling you have when you are scared and that the rest of the time you are not afraid. Jesus says the entire world is made of fear which is the opposite of love, all states of sickness including death are fear of awakening, and fear includes every state of the ego thought system.
Sin. You think it means something you did. Jesus redefines it as something you think you did which you didn't do in reality. Sin is the authority problem. It's reversal of cause and effect, opposing God's will, which has produced an entire world of sin and death. "Without the idea of death there is no world." In sin, the death wish is formed, making death inevitable, unless you undo sin.
Love. You think of special love, an emotion of love, private love, exclusive love. Parental love. Small amounts of love. Jesus says all expression of real love are maximal - maximum total unconditional whole-minded full-hearted unlimited universe-sized love.
Forgiveness. You think it means pardon. Or you think it is a process. Jesus states it is a constant state of mind in which you recognize there is nothing left to forgive because you have not condemned yourself. All forgiveness is self forgiveness, yet you are one with others. Forgiveness is the atonement.
Ego. You think ego is just some little bit of egotism or selfishness. Jesus redefines ego as the separation idea and an entire thought system which has manifested as a physical universe, complete with dream bodies and war and death.
Perception. You think perception is you being inside your body peering out through your eyes seeing stuff in a certain way. Yet Jesus says the world is perception, physical objects are mental images, the whole world is your perception within your universe-sized mind, and it's not so much to do with how you see something that's out there. Perception is a result, not a cause. Projection makes what you see.
There's many more things which we do not recognize as being what they are. And so it may be easy to mis-read what Jesus means by certain words. We have to unlearn all of our previous definitions for things and broaden the scope of them. Jesus is accurate and truthful in HIS understanding of what things ARE, he knows what is fear and what is death and what is life, in the most accurate of ways. Our definitions are limited and basic and incorrect. Dictionary definitions are also flawed. This is part of what makes learning ACIM challenging, he's using familiar words in unfamiliar ways.