From Longman Dictionary of Contemporary Englishgo off at a tangentgo off at a tangentBritish English, go off on a tangent American English informalTHINK/HAVE THE OPINION THAT to suddenly start thinking or talking about a subject that is only slightly related, or not related at all, to the original subject Let’s stay with the topic and not go off at a tangent. → tangent
Examples from the Corpus
go off at a tangent• As for going off at tangents, my dear, I do it myself, hormone balance not withstanding.• Loretta's mind went off at a tangent. 