In a manner that descends or goes downward; also, in a condescending or patronizing way toward someone considered inferior.
From descending (present participle of descend) + -ly adverbial suffix. The 'condescending' meaning evolved as a metaphorical extension of downward movement.
The word beautifully captures how language embodies physical actions as moral judgments—'looking down on' someone literally and looking down on them metaphorically merged into one word.
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