any object or person whose name you have temporarily forgotten or do not know

Where's the whatsit that you change channels with? 你换频道用的那个叫什么的东西哪儿去了？

You'd better tell whatsit - what's his name - the guy in charge of stationery. 你最好告诉那个人——他叫什么名字来着——就是那个负责管文具的人。