Opposed to or rejecting established customs, traditions, and normal social practices.
From anti- (against) + conventional (from convention, from Latin convenire, to come together, + -al, adjective suffix). This term became increasingly common in the 19th-20th centuries as artistic and social movements deliberately challenged tradition.
The Romantic poets, jazz musicians, and beatnik writers were all anticonventional—but it's interesting that being against convention requires convention to exist first; you can't rebel against rules nobody follows.
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