Meaning:
“桑拿” is transliteration of the word “sauna,” and “天” is short for “天气,” which means “weather.” Chinese people use the term to refer to the dog days of summer, when it’s so hot and humid that it feels like you’ve entered a sauna cabin.
Example:
A: 今晚有空出来玩吗？
Jīnwǎn yǒukòng chūlái wán ma？
How about hanging out with me this evening?
B: 好啊。可是这种桑拿天干点什么好呢？
Hǎo a。Kěshì zhèzhǒng sāngnátiān gàn diǎn shénme hǎo ne？
OK. But what can we do in this terribly hot and humid weather?
A: 我们去看电影吧，电影院里有空调。
Wǒmen qù kàn diànyǐng ba，diànyǐng yuàn lǐ yǒu kōngtiáo。
Let’s go to the movies. The theaters are air-conditioned.
B: 好主意。
Hǎo zhǔyì。
That’s a good idea.