noun
energy; vitality; mental determination
Refers to one's total physical and mental energy, often used in contexts of exhausting or pouring all one's strength into something. Common in set phrases like 精根尽きる (to be completely exhausted) and 精根を傾ける (to devote all one's energy).
彼は研究に精根を傾けた。
He devoted all his energy to his research.
長い闘病生活で精根尽き果てた。
After a long battle with illness, he was completely drained of energy.
Compound of 精 (spirit, energy) and 根 (root, stamina). The exact historical derivation is uncertain, but the combination naturally conveys the idea of fundamental energy or life force.