When a place is airless, the air inside feels still and heavy, as if no window has been opened for a long time. A shut classroom after lunch, a small bedroom in summer, a crowded underground station — you may feel sleepy, warm in an unpleasant way, or a little sick, because fresh air is not moving. In everyday talk many people just say stuffy; airless is a little more like a book, a careful description, or a complaint said slowly. You can speak of an airless room, an airless night, or an airless atmosphere (the feeling in a room, not the sky). Though actually, scientists also use airless for a place with no air at all, like the moon: nothing to breathe, no wind. That is the same idea stretched to the end — not “bad air,” but no air. The meaning you meet first is the stuffy one. It is not a rude word, and it is not “air that is less,” as if you were doing maths.

不通風的、悶的