In a gleaming manner; with a bright, shining quality or luminous appearance.
From 'gleaming' (present participle of 'gleam' or adjective form) plus the adverbial suffix -ly. This double-suffix construction (gleam + -ing + -ly) is common in English.
Words like 'gleamingly' show how English can layer suffixes—one word can have multiple layers of meaning-building, where each suffix adds a layer, like 'shine' → 'shining' (present) → 'shiningly' (how it appears).
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