What is the meaning of 當 and 自強 in 男兒當自強?
男兒 is MAN

當 is 應當， which means SHOULD，MUST

自 means 自己， SELF

強 is a verb short for 圖強，here 圖 means FIND WAYS TO DO STH，and 強 means TO BE STRONG.

So 男兒當自強 means A man should find ways to make himself strong