Superlative form of flossy; the most resembling floss, the most fluffy, or the most flashy and showy.
From 'flossy' plus the superlative suffix '-est', which is the standard way English forms the highest degree of comparison for adjectives.
The '-est' superlative ending is ancient Germanic grammar that English inherited, so when you say 'flossiest' you're using a grammatical pattern that's over 1,500 years old!
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