Spring silkworms will only run out of silk when they die [chūn cán dào sǐ sī fāng jìn]

Basic definition

Si: a pun, a homophony of "thinking". It is a metaphor for a deep friendship that lasts until death.

Source

Poetry "Untitled" by Li Shangyin of the Tang Dynasty: "The silk will not be used up until the spring silkworm dies, and the wax torch will turn to ashes and tears will dry up.

Explanation of the idiom: Silk : A pun, a homonym for "si". It is a metaphor for a deep friendship that lasts until death. Now it mostly refers to a spirit of devotion that lasts until death.