sit in on (a meeting/a class) 旁聽

說明：

sit in 指「參加」、「出席」、「觀察」，後接 on (an event)。其相反詞是 sit out (a game)。所以「休憩處」就是 sitting-out area。

例子：

Qualified members can sit in on Tom's lessons.

by Thomas Shum