Full of beams; shining brightly like beams of light, or resembling beams structurally.
From 'beam' plus the adjectival suffix '-y' (from Old English), a productive pattern for creating adjectives meaning 'having the quality of' or 'full of' (like dreamy, creamy, steamy).
The '-y' suffix is one of English's oldest and most useful: sunny, rainy, funny, beamy. It can transform almost any noun into an adjective, and it's been doing this job since before English had a written standard.
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