From 资治通鉴: a minister pleads 群臣或从陛下岁久，身经百战，所得不过一州, 'some of your ministers have followed you for years and fought a hundred battles, yet received only a single prefecture.' Originally literal military experience.
usage
Modern use has expanded far beyond the battlefield, seasoned sales reps, experienced programmers, hardened negotiators. Positive and admiring. Often with 的 as a modifier: 身经百战的老将 / 老兵 / 销售.
