not

莫 is composed of the BuShou of 艸(grass), 日(the sun) and大, 大 should be the variant of 艸, so 莫 depicts the sun being covered by grass. In the evening, the sun goes down and almost reaches horizon, so low that even being covered by grass. In the past, 莫 had the meaning of bushy(for example, in The Book of Songs(詩經), a verse written ‘維葉莫莫’), 莫 also means darkness or dusk, but such meaning has been replaced by 暮. On the other hand, ancient Chinese like brightness but hate darkness, they wish the sun will not go down, so 莫 has the meaning of ‘Don’t’.