Opposed to, hostile toward, or lacking the qualities of music; not having pleasant or harmonious sound.
From anti- (against) + musical (from music, derived from Latin musica, itself from Greek mousike meaning 'of the Muses'). The prefix anti- negates or opposes the concept that follows it.
The 20th-century composer John Cage created 'antimusical' pieces—like his famous 4'33" of silence—which challenged what counts as music and forced listeners to realize that the absence of organized sound can be more powerful than notes themselves.
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