No, it's a Chinese lesson.

A "thing" or "stuff" in Chinese is 东西 (dōngxi). That's literally "east-west" if you pick the individual characters apart. That's what the footnote refers to.

Calling somebody 不是个东西 (bùshì ge dōngxī) means something along the lines of being good for nothing, i.e. an insult. Translating it literally, it would be calling somebody "not a thing".