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Phrase(s): *on a fool’s errand
Fig. involved in a useless journey or task. (*Typically: be ~; go ~.) • Bill went for an interview, but he was on a fool’s errand. The job had already been filled. • I was sent on a fool’s errand to buy some flowers. I knew the shop would be closed by then.
McGrawhill's American Idioms And Phrasal Verbs