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Phrase(s): *off on a sidetrack
Fig. on a digression; discussing a topic that is not the main topic. (Alludes to a train waiting on a siding. *Typically: be ~; get ~; get someone ~.) • Anne got off on a sidetrack and never returned to her topic. • The ineffective committee got off on one sidetrack after another.
McGrawhill's American Idioms And Phrasal Verbs