An Etymological Dictionary of the German Language/oft
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oft, adv., from the equiv. MidHG. oft, ofte, OHG. ofto, adv., ‘often, frequently’; corresponding. to Goth. ufta, OIc. opt, AS. oft, E. oft (extended form often), OSax. oft, ofto, ‘often.’ These adv. forms seem to be petrified cases of an obsolete subst. or adj. partic.; they have also been connected with the partic. of the Sans. root uc, ‘to be fond of doing.’