If "affirm" means to declare something to be true, then the opposites are:
* Deny: To declare something to be false.
* Contradict: To state the opposite of something.
* Disprove: To show that something is false.
* Refute: To prove something to be wrong.
* Negate: To cancel out or make ineffective.
If "affirm" means to express agreement or approval, then the opposites are:
* Deny: To refuse to agree or acknowledge something.
* Disagree: To express opposition to an opinion or statement.
* Disapprove: To express disapproval or lack of approval.
* Reject: To refuse to accept something.
If "affirm" means to strengthen or confirm something, then the opposites are:
* Weaken: To make something less strong or effective.
* Undermine: To weaken the foundation or support of something.
* Debilitate: To make something weak or infirm.
* Dispute: To challenge or question something.
The best opposite for "affirm" will depend on the specific meaning in your context.