on a tangent

Idioms  

1. 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, 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]

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Example Sentences

They briefly go off on a tangent, imitating weather reports and station IDs for the British Forces Broadcasting Service — the kind of reference that in pre-internet days required listening to this year’s message over and over again to try to decipher exactly what they’re talking about.

From Salon

The first rule of making one’s point was “Stick to the subject at hand,” but the admiral was already off on a tangent and she would have to steer the conversation back to port, so to speak.

From Literature

To go off on a tangent is always a risky maneuver, for once one has gone, it is often surprisingly difficult to find one’s way back.

From Literature

Ordinarily Penelope would have been very proud of his pteridomaniacal expertise, but at the word “letter” her mind had skipped off on a tangent from which it had not yet returned.

From Literature

To seriously consider the answers to these questions would require Penelope to do something called “going off on a tangent,” which is another way of saying “to stray from the subject at hand.”

From Literature