adverb
just how; how (dare you)
Emphatic adverbial expression used to express surprise, indignation, or admiration at someone's audacity or ability. Often carries a tone of disbelief or reproach.
よくもまあそんなことが言えるね。
How dare you say such a thing!
よくもまあ一人でここまで来られたものだ。
I'm amazed you managed to come all this way by yourself.