In a frothy, foamy, or bubbly manner; in an insubstantial, frivolous, or excited way.
From 'frothy' plus the adverbial suffix '-ly.' The -ly suffix converts adjectives to adverbs describing how something is done.
Adverbs ending in '-ly' are so common that English speakers can add '-ly' to almost any adjective—it's one of the most productive word-building patterns in the entire language.
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