A silly or stupid mistake, or a goofy (silly and clumsy) person.
Originally 'goff' or 'gooff' in the 1500s, possibly from Middle English 'gof' (meaning a simpleton). The spelling shifted to 'goof' by the 1600s, and Disney's character Goofy (created 1932) crystallized the modern meaning of silly and lovable rather than cruel.
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