Lacking a cloister or without the characteristics of a cloister; not confined or secluded.
Cloister plus the suffix -less meaning 'without' or 'lacking.' This is a productive suffix in English that turns nouns into adjectives describing absence (homeless, careless, fearless).
Adding '-less' to nouns is one of English's most powerful word-building tricks—it instantly creates an opposite meaning. This particular word is quite rare, suggesting that most people just say 'open' or 'secular' instead.
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