seriously ill patient; critical case
Refers to a patient whose condition is severe or life-threatening. Often used in medical or hospital contexts.
Several seriously ill patients are hospitalized at that hospital.
The doctor did everything possible to treat the critical patient.
Compound of 大 (dai, 'big, serious') + 病人 (byōnin, 'sick person'). The exact historical derivation is uncertain, but the meaning is transparent from its components.