utterly mortified; completely upset; highly regrettable
A strong, somewhat formal yojijukugo expressing deep personal chagrin, mortification, or aggrieved disappointment at a situation or someone's actions. Often used when one feels wronged or let down.
I am utterly mortified by his irresponsible attitude.
It is highly regrettable to be misunderstood in such a way.
心外 means 'unexpected and regrettable' or 'mortifying', but 心外千万 intensifies this to an extreme degree, adding the sense of being completely or utterly so.
遺憾 is a formal term for 'regrettable', often used in official statements. 心外千万 is more personal and emotional, focusing on one's own mortification rather than objective regret.
A yojijukugo combining 心外 (unexpected and regrettable) with 千万 (extremely, myriad), literally 'a myriad of unexpected regrets', emphasizing an intense feeling of mortification.