Not wearing a belt, or lacking a belt; can also describe something that doesn't have a belt-like structure or feature.
From 'belt' plus the suffix '-less' meaning 'without.' This is a straightforward English formation following standard rules for creating negatives.
Going beltless is actually a fashion statement that keeps cycling back—from the 1920s flappers to modern minimalism, people reinvent the beltless look every few decades as a form of freedom and style!
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