Why did the Japanese change the meaning of the Chinese character from treasure to penis!

I studied English with a Japanese classmate. During the break, I explained to him the interesting parts of Chinese characters. For example, "屌", "尸" means body, and "吊" means hanging. So "屌" is something hanging from the lower part of the body. He wrote two words for me: "珍宝". He said that these two words are used in Japanese. How come treasure becomes penis in Japan? But it makes sense. This thing is really precious.