how admirably; how wonderfully; how skillfully
An exclamatory phrase expressing admiration or amazement at someone's skill, quality, or achievement. Often used with a tone of impressed surprise.
本当にまあ、よくできましたね。
My, how wonderfully well you've done it!
本当にまあ、見事な演奏だった。
Really, what a splendid performance that was.
A fixed phrase combining the adverb 本当に (truly) with the interjection まあ (oh, my). The exact origin as a set expression is uncertain, but it likely developed as a natural emphatic exclamation.