In a manner characterized by fountains; abundantly and flowingly; in the style or manner of a fountain.
From fountainous (resembling fountains) + -ly suffix forming adverbs. The -ly suffix comes from Old English -lice and transforms adjectives into adverbs describing how something happens.
This adverb is so rare it's almost disappeared from English, but when you read older poetry, you might find language described as flowing 'fountainously'—music to Victorian ears!
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