In a manner that is excessively artistic or pretentiously concerned with art; in an overly self-conscious artistic way.
From arty (adjective, meaning 'affectedly or pretentiously artistic') plus -ly (the adverb-forming suffix). Arty itself emerged in the late 19th century as a colloquial term.
The word 'arty' became popular as a way to mock people who talk about art too much without really understanding it—it's basically the linguistic equivalent of an eye-roll that became so useful it stuck around.
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