na-adjective
extremely regrettable; deeply unfortunate
Formal, literary yojijukugo expressing strong regret or disappointment. Often used in official statements or serious contexts.
今回の事件は遺憾千万だ。
This incident is extremely regrettable.
遺憾千万な結果に終わった。
It ended in a deeply unfortunate result.
Yojijukugo combining 遺憾 (regrettable) and 千万 (extremely, myriad). The exact historical derivation is uncertain, but it follows a pattern of intensifying compounds.