Without air; stuffy and lacking fresh air circulation, or existing in the absence of air (like in space).
From 'air' + '-less' (suffix meaning 'without' or 'lacking'). The '-less' suffix makes a direct opposite, creating an antonym of 'airy.'
The Moon is called 'airless' even though technically it has an extremely thin exosphere—it's one of those words where we use linguistic shortcuts because 'practically without breathable air' is too long to say every time!
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