on a tangent

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]

Example Sentences

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.

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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.”

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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.

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“But that is quite enough wondering about that,” she told herself, for they had arrived at the gates of the palace, and it was no time to go off on a tangent.

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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.

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