smooth, vb.; *smoothen.
The latter is a needless variant — e.g.: "But eventually, Dr. Toaff insists, the knobs and bulges will smoothen [read 'smooth'] out." Natalie Angier, "One Woman’s Decision Against a Hysterectomy," N.Y. Times, 18 Feb. 1997, at C1.
The verb is often misspelled *"smoothe," doubtless on the analogy of "soothe" and "teethe" — e.g.: "They are also using a polishing process that smoothes [read 'smooths'] the metal that sits above the circuitry." Benjamin Fulford, "Another Dimension," Forbes, 22 July 2002, at 173. This misspelling may also be influenced by the past-tense form: "smoothed."
Language-Change Index — *"smoothe" for the verb "smooth": Stage 1.
*Invariably inferior forms.
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Quotation of the Day: "The most common fault of writing is wordiness; the most important editorial job is cutting." Rudolf Flesch, "How to Raise Readability," in Best Advice on How to Write 78, 80 (Gorham Munson ed., 1952).