lit. to look at the ocean and let out a sigh

When something that you experience is so large and immense that you feel small or frustrated. So for example, you want to learn Chinese but you see the thousands of Chinese characters that you have to memorize so you say 望洋興歎, as in "there's so many characters, I can't memorize them all, it's such a pity."

Or you are learning guitar and you think you are pretty good, but then you meet somebody who is an absolute master at it and you can do nothing but 望洋興歎.