Literally: drunk – life – dream – death
Alternately: Idling one’s life away. Wasting one’s life. Living as if drunk; dying as if in a dream.
Notes: Apparently some people write the homophone 無死 instead of 夢死. This is an error, as the former is a modern term referring to having no outs in baseball!
The source on this is a bit confusing, but it seems to come from an elementary-level textbook called the 小学 (Xiaoxue) from 12th-century China, in which the saying is attributed to Cheng Yi in a section called 嘉言, “wise sayings.”