“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.
Jīnwǎn yǒukòng chūlái wán ma？
How about hanging out with me this evening?
Hǎo a。Kěshì zhèzhǒng sāngnátiān gàn diǎn shénme hǎo ne？
OK. But what can we do in this terribly hot and humid weather?
Wǒmen qù kàn diànyǐng ba，diànyǐng yuàn lǐ yǒu kōngtiáo。
Let’s go to the movies. The theaters are air-conditioned.
That’s a good idea.