This is the 1976th most frequent Japanese word.
なにせ
"なにせ" means "in any case" or "anyhow."
なにせ here emphasizes the urgency and provides a context for the given situation.
なにせ早く行かないと間に合わない。
Anyway, if we don't go quickly, we won't make it in time.
なにせ in this sentence introduces the reason for not participating in a dismissive or explanatory way.
そのイベントには参加できません、なにせ忙しいので。
I cannot participate in that event, for I'm too busy.
なにせ is utilized to emphasize the strong reason behind her immediate action.