In the old days, rue was placed in books to keep insects away.
Tips
usage
Most common today in 芸芸众生 (yúnyúnzhòngshēng, all living beings, the common multitude). The original sense is the rue herb; because rue repelled insects, 芸 also became a poetic word for books and libraries.
云 (yún, cloud) sits below and gives the exact sound with no tone change. Its scattered-cloud image also fits the sense of a vast, countless multitude in 芸芸.
