断然 + negation (不/没/无) = 'absolutely not' (very formal/written). 断然 + verb of refusal/decision = 'resolutely'. The 断 means 'cut off' - the metaphor is one decisive cut, no further discussion.
register
Formal and slightly literary. In speech most people say 坚决 or 绝对. 断然 turns up in news writing, editorials, and diplomatic statements.
