This is the 603rd most frequent Japanese word.
決して
"Never" or "by no means" (used with a negative verb).
This sentence uses "決して" to mean "never" in the sense of an emphatic negation.
決してあきらめない。
I will never give up.
Here, "決して" conveys strong determination or an absolute standpoint in denying forgiveness.
その犯人は決して許されるべきではない。
That criminal should never be forgiven.
In this sentence, "決して" modifies the verb to clearly emphasize the prohibition or strong advice against an action.