Without gleam; lacking brightness, shine, or luster; dull or non-reflective.
From 'gleam' plus the negating suffix -less, which means 'without' or 'lacking.' This productive suffix appears in hundreds of English words (homeless, careless, fearless).
The -less suffix is fascinating because it transforms 'gleam' into its opposite—but unlike 'not gleaming,' which is passive, 'gleamless' makes dullness sound like an active property, which is why it's more common in poetic descriptions.
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