Present participle of 'gussy,' the action of dressing up or decorating something fancily.
From 'gussy' with the standard English progressive ending -ing, maintaining the American slang origins of the base verb.
The -ing form allows English speakers to talk about actions in progress or as nouns, so 'gussying up' describes the actual process of making yourself fancy—it's the action itself, not just the result.
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