釋氏要覽 3.33

3.33

肆者。所以陳貨鬻之物也。因後漢張揩。字公超學徒隨之。所居為市故。今學處稱肆焉。

A si (market stall) is a place where merchandise is displayed and sold. Because Zhang Kai, styled Gongchao, of the Later Han had disciples following him wherever he stayed, his dwellings became marketplaces; hence, places of learning are nowadays called si.”