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.
In the 1960s, 'goof' became slang for laughing or being silly ('to goof around'), and this linguistic shift coincided with counterculture attitudes that celebrated goofiness as a form of rebellion against seriousness — a reminder that what seems like new slang is often just old words finding new social energy.
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