on a tangent
On a sudden digression or change of course, as in The professor's hard to follow; he's always off on a tangent. This phrase often occurs in the idioms fly off or go off on a tangent, as in The witness was convincing until he went off on a tangent. This expression alludes to the geometric tangent—a line or curve that touches but does not intersect with another line or curve. [Second half of 1700s]
Words nearby on a tangent
on a pedestal, put,
on a rampage,
on a roll,
on a shoestring,
on a string,
on a tangent,
on account,
on account of,
on again,
on all fours,
on an even keel