Literary word meaning the entirety of one's life. Often used in set phrases like 畢生の大作 (one's life's work) or 畢生の事業 (one's life's enterprise).
This painting is his life's great work.
He devoted his entire life to this research.
A borrowing from Chinese 畢生, literally one’s complete life or all one’s life. 畢 carries the sense of completion or entirety, and 生 means life.