The present participle of 'beetle,' meaning moving quickly and nimbly; or as an adjective, protruding or overhanging in a threatening manner (as a beetling brow).
From 'beetle' (the verb, meaning to move quickly or jut out) plus the '-ing' suffix, which can create either gerunds/present participles or adjectives, especially when describing appearance.
A 'beetling brow' in Shakespeare and Victorian literature became a shorthand for evil—the idea that overhanging eyebrows blocking the eyes suggested a mind turned inward toward dark thoughts.
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