To regret in old age the time wasted when one was young; refers to the uselessness of feeling sorry about one's laziness or missed opportunities later in life. This is the second half of the couplet 少壮不努力，老大徒伤悲 shào​zhuàng​bù​nǔ​lì​lǎo​dà​tú​shāng​bēi