Money, coins.

This medieval Chinese term refers to the fact that the traditional Chinese coin (one "cash") has a square hole in the middle.

The term "Brother Square-Hole" (Kong1-fang1 xiong1) originates with Lu3 Bao1, a Jin dynasty author, whose "Essay on the God Cash" contains this description:
The body of Cash
bears the Signs of Qian and Kun
: Internally he is square;
externally he is round.

...He is deified and treasured by all the world.
We call him elder brother, as though kin;
his courtesy name is Square-Hole.

The Song dynasty poet Huang Tingjian has a line describing being out of money:

"Brother Square-Hole writes a letter breaking off relations".

Other Chinese literary allusions