Covered with or resembling glitter; sparkling with brightness or small flashes of light.
From glitter + -y (adjective-forming suffix). This became the standard English adjective form by the 19th century, originally used to describe objects decorated with actual glitter particles.
Glitter itself (the noun) is a relatively young word—it wasn't widely used for the decorative particles until the 20th century, yet 'glittery' feels like it's been around forever. It's a perfect example of how English can quickly turn any vivid concept into an adjective.
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