In a manner that rejects or opposes established social customs and traditions.
From anticonventional + -ly (adverb suffix). The -ly ending transforms adjectives into adverbs describing how actions are performed, a productive pattern in English since Middle English.
When someone dresses anticonventionally or speaks anticonventionally, they're making a choice to reject what's expected—but language-wise, you need the conventional word 'conventional' to even create the anti-version, so convention always wins linguistically.
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