expression, godan verb (-ru)
to be on the brink of death; to be at death's door
A formal, literary expression meaning death is imminent. Rare in modern conversation; used in historical or dramatic contexts.
彼は重い病で、命旦夕に迫っていた。
He was seriously ill and on the brink of death.
A classical Chinese-derived expression. 旦夕 (たんせき) means 'morning and evening', implying a very short time. The phrase literally means 'life presses upon morning and evening', i.e., death is imminent.