The definition of fly off on a tangent is:
(verb) to start talking about an unrelated topic
The students complained about the professor, because he always flies off on a tangent and starts talking about unrelated things.
While arguing about whose turn it was to do the dishes, Shannon flew off on a tangent and started discussing the laundry.
You are always flying off on a tangent when we’re speaking! Stick to the topic!
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