fly off at a tangent

To begin addressing or discussing a topic that is different than or not relevant to the main discussion. I tried to address the customer's problem, but she kept flying off at a tangent and I couldn't understand what her true complaint was. In the middle of our conversation about my finances, my advisor flew off at a tangent about current events. All of a sudden, in the middle of dinner, Tom flew off at a tangent about his time in Peru.

go/fly off at a ˈtangent

(British English) (American English go off on a ˈtangent) change suddenly from talking or thinking about one thing to talking or thinking about another: One moment the professor is working hard on a problem in physics, the next he’s gone off at a tangent and he’s talking about bees.

A tangent is a straight line that touches the outside of a curve but does not cross it.