in a manner resembling glass; with a smooth, glassy, or emotionless appearance or quality.
From 'glassy' (adjective) plus the adverb-forming suffix '-ly.' 'Glassy' comes from 'glass,' ultimately from Proto-Germanic roots meaning to glow or shine.
When someone stares 'glassily' into space, you can see the gloss on their eyes just like looking at smooth glass—it's one of the few adverbs that describes an actual visual appearance!
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