Could someone explain the ironic(?) use of よくいうよ?

Question

Dainichi was kind enough to tell me that よくいうよ means "to shamelessly say something you should be ashamed to say."

On spacealc, this explanation and translation is provided:

よく言うよ。 What are you saying?

〔相手の発言の矛盾を皮肉っぽく批判する言い方。「何を言っているんだ？」が直訳で、「そんなことを言うなんて信じられない」というニュアンスを含んでいる。〕

What meaning does this particular よく provide, and which kanji, if any, does this particular よく derive from? (I am interested in the literal meaning of this よく from which the figurative meaning derives i.e., does it mean "talk well," "talk brazenly," etc)

Answer 1

goo, よく【善く／良く／▽好く／▽能く／▽克く】

４ 困難なことをしたり、考えられないような喜ばしい結果を得たりして感じ入るさま。本当にまあ。よくぞ。「―来てくれました」「月給だけで―やっていけるね」

4 Being impressed by someone doing something difficult or achieving a miraculous result. (alternates). (examples)

５ 相手の非常識な言動などを非難するさま。4を反語的にいう語。よくもまあ。「―のこのこと来られたものだ」

5 Reproaching a person's absurd words/deeds (and the like). Language expressing 4 ironically. (alternate). (example)

Those are your kanji, at the top; 5 is your meaning; and 4 is what it derives from (according to goo). (I don't know that 5 is "figurative" per se; I think it's just sarcastic.)

Answer 2

I remember this as the kind of thing a bar hostess would laughingly say to a patron who just made a joke. With the right intonation and context these "I'm impressed/appalled" phrases can be jovial and light-hearted (and obviously dangerous for us to use).