seriously ill patient; critical case
Refers to a patient whose condition is severe or life-threatening. Often used in medical or hospital contexts.
その病院には大病人が何人も入院している。
Several seriously ill patients are hospitalized at that hospital.
医者は大病人の治療に全力を尽くした。
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.