expression, godan verb (-u)
practice virtue; act virtuously
A somewhat formal or literary expression meaning to consciously perform virtuous deeds or lead a moral life.
彼は生涯を通じて徳を行い、多くの人に尊敬された。
He practiced virtue throughout his life and was respected by many.
More common and casual way to say 'accumulate good deeds'; 徳を行う is more formal and philosophical.
Compound of 徳 (virtue) and 行う (to perform). The exact historical origin is uncertain, but it follows a standard Sino-Japanese pattern for expressing moral conduct.