Completely; totally; absolutely. Often used with negation to mean 'not at all.'
彼の言っていることはまるっきり意味が分からない。
What he says makes absolutely no sense to me.
この二つはまるっきり別の問題だ。
These two are completely different issues.
準備不足で試験はまるっきりダメだった。
Due to lack of preparation, the exam was a complete disaster.