、、。
Oh my, really, I had absolutely no idea.
Literal
Oh-my, really, at-all didn't-know.
A small exclamation with one notable feature. 知らなんだ is an archaic/regional dialect form of 知らなかった ('did not know') — the ~なんだ ending is a classical/Kansai-dialect past negative, still heard in some regional speech and in storybook, anime, or fictional character dialogue. Modern standard Japanese uses 知らなかった universally. Hearing/seeing 知らなんだ marks the speaker as using deliberately old-fashioned, regional, or character-stylized speech. あらまあ is a feminine exclamation of surprise — 'oh my,' 'my goodness' — typical of older female speakers or female anime characters.