go off on a tangent

US /ɡoʊ ɔf ɑn ə ˈtæn.dʒənt/
UK /ɡoʊ ɔf ɑn ə ˈtæn.dʒənt/
"go off on a tangent" picture
1.

to suddenly start talking or thinking about a different subject from the one you were originally discussing

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The professor tends to go off on a tangent during his lectures, making them hard to follow.
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We were discussing the budget, but then he suddenly went off on a tangent about his vacation plans.
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go offgo off withgo off the railsgo off the deep endoffoff and ongo