This is the 892nd most frequent Japanese word.
否定
Denial; negation.
In this sentence, "否定" is used as part of the compound word "自己否定的," which means "self-denying." It qualifies his way of thinking or perspectives.
彼は自己否定的な考えを持っている。
He has a self-denying attitude.
Here, "否定" is used as a verb (via its base noun form) to express the action of denying or contradicting the truth of a theory.
その理論が正しいことを否定する証拠はない。
There is no evidence to deny the correctness of that theory.
In this context, "否定" is part of the adjective "否定的," meaning "negative" or "contradictory," describing the nuance carried by her words.