天年 (tiānnián) is the lifespan Heaven ordained for you — in Confucian and Daoist thought, ideally around the canonical 'three score and ten' or beyond. To
不终天年 is to be cut down by violence, illness, or war before reaching that allotted span. The phrase carries a sympathetic, slightly fatalistic tone — see also the antonym
颐养天年 'to live out one's natural years in peace'.
