More frowsty; more warm, stuffy, and close (of an atmosphere), or more inclined to frowsting.
Comparative form of 'frowsty', created with the comparative suffix '-er', where 'frowsty' derives from 'frowst' (the verb for lounging in a stuffy room).
Comparing 'frowstier' rooms is a very British pastime—it's the kind of word you'd use to describe which pub felt more cozy and warm versus which one was uncomfortably close and stuffy.
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