From Yue Fei's (岳飞, Southern Song) 《满江红》: 莫等闲，白了少年头，空悲切。(Do not idle — lest youthful hair turn white, and grief come in vain.) Written by the general as he urged himself and his men not to waste the chance to reclaim lost northern territories from the Jurchen Jin.
usage
Almost always quoted with the preceding 莫等闲 and following 空悲切. Common motivational line for students, entrepreneurs, and anyone tempted to procrastinate.
