The words deny, reject, refuse and decline are often confused. Of these, the words reject, refuse and decline have very similar meanings. Interestingly, the word accept can be the opposite of all of them. Deny has a different meaning. The opposite of deny is admit. To deny is to say that something is not true. To deny somebody something is to refuse to give it to them. He denied taking the money She denied that she had acted irresponsibly. The minister denied the allegations. She denied him the right to visit her daughter. She denied me the permission to go. The guards were accused of denying the prisoners food and water. To refuse to do something is to say that you won’t do that. She refused to clean the floors. The child refused to take the medicine. She refused to pay the dues. When you refuse something you do not accept it. I offered her a drink but she politely refused it. They refused the invitation. Reject is quite similar to refuse. To reject a proposal is to refuse to accept it. The promoters rejected the plan out of hand. The manager rejected my request for more time to finish the project. If someone rejects you, they show indifference towards you. She was rejected by her own people. Reject can also be used as a noun. The noun reject is used to refer to something or somebody that has not been accepted. The gown was very cheap because it was a reject. Decline is relatively formal. It has more or less the same meaning as refuse. To decline something is to politely refuse to accept it. I offered her a job but she declined it. She offered to bear all the expenses but he politely declined. Decline can also mean become less or become worse. The quality of life in many of our cities is declining. The number of people who buy homes have declined in recent years.

I'm not willing to obey your order.

"I refuse your order" would most likely mean that I will not agree to accept or receive your order from you. For example, you hand me an order form and I won't take it from you. Another example: I might refuse a package by telling the delivery person to take it away. "I reject your order" could be used the same way, but the word reject suggests that I looked at your order, considered it, and decided to say no. Maybe I accepted your order form, but when I looked at the form, it had several errors, so I rejected it. I will not honor or agree to the request.

Both mean you do not receive or accept something (a physical thing or an idea, suggestion, etc.).  The difference in Grammar: "Refuse" is often followed by an infinitive (to + VERB). Reject usually has a noun or noun phrase after it. For example, I refuse to return your pen. And, I reject your suggestion to return your pen.

Deny also has a similar meaning to refuse and reject.