I'm curious what you're reading! That's an old expression. When it (usually the weather or the air around you) is "close," it's humid, probably slightly warmer than you'd like, and maybe a bit musty-smelling if it's indoors... Where I live, that word was only used by my grandmother's generation. So, despite its meaning, it brings back nice memories of her in the summer! HTH. Chris (italki teacher https://www.italki.com/chrisbreakspear)

"close" is used to describe a hot, sticky, unhealthy atmosphere. So "close-smelling" is probably similar to "stale", in this context. It's not a word I've ever seen used.