impure blood; old blood
Refers to blood that is considered impure or stagnant, often in a traditional or folk-medicine context. The term is not common in modern medical usage.
昔の人は、古血が体に悪いと考えていた。
People in the past believed that impure blood was bad for the body.
Compound of 古 (old) and 血 (blood). The exact historical derivation is uncertain, but it likely originated in pre-modern medical or folk beliefs about stagnant or impure blood.