What does 老大 mean in 少壮不努力, 老大徒伤悲?
老大 here is not the noun 'leader/ boss'
It is an adjectives phrase
老 = old; 大 = grown (in age)
'老大' in '老大徒伤悲' means &quot;old&quot;
Detail break down of this expression:

少壮 - young and in your prime

不努力 - not work hard

老大 - grown and old

徒伤悲 - only have sadness


少壮不努力, 老大徒伤悲 means &quot;If you don't work hard when you are young, there will only be sadness for you when you get old&quot;