go/fly off at a tangent

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.
See also: fly, off, tangent