Exactly the opposite; completely contrary — used to emphasize that something is the exact reverse in nature, opinion, position, etc.
彼の考えは私の意見と正反対だ。
His opinion is exactly the opposite of mine.
二つの報告書は結論が正反対だった。
The two reports had completely opposite conclusions.
その行動は期待とは正反対の結果を招いた。
That action produced a result completely contrary to expectations.