To make something overly complicated or fussy; to add unnecessary complications or details to something simple.
From 'fuss' plus the suffix '-ify' (meaning 'to make or cause to become'), allowing 'fuss' to function as a verb of transformation.
The '-ify' suffix is incredibly productive in English—we can apply it to almost any noun or adjective to create new verbs, which is why speakers can understand and use words they've never heard before.
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