extremely regrettable; deeply unfortunate
Formal, literary yojijukugo expressing strong regret or disappointment. Often used in official statements or serious contexts.
This incident is extremely regrettable.
It ended in a deeply unfortunate result.
遺憾 is a noun/adjectival noun meaning 'regrettable'; 遺憾千万 is a stronger, more emphatic yojijukugo form.
残念 is a common, everyday word for 'unfortunate' or 'disappointing'; 遺憾千万 is much more formal and emphatic.
Yojijukugo combining 遺憾 (regrettable) and 千万 (extremely, myriad). The exact historical derivation is uncertain, but it follows a pattern of intensifying compounds.