To have an image of bamboo in one's mind
Wang Xi-zhi is one of the most famous calligraphers during the Eastern Jin Dynasty of China. When he was very young, he practiced his art every day and never stopped.
Later he absorbed the strong points of all the other schools of calligraphy, and developed his own unique style of writing. Because of his achievements, he has been honored as one of China's sages of calligraphy.One time, Wang Xi-zhi sketched in wood for an engraver to cut. Then the engraver found the ink had penetrated one centimeter into the wood.
"Ru Mu San Fen" is got from this story, which means the calligraphy is penetrating.
Now it is often used to describe expressing sharp ideas or profound views.