Idiom (UK), see go off on one. To launch into a rant or lengthy discussion about some pet subject.

Might be derived from "going off on a tangent", but has more of a connotation of annoyance.
Sorry for going off on one, but I really feel strongly about this.
by Cedders February 27, 2011
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To be provoked by someone/something to such a point that a verbal reaction becomes inevitable.
Why's that kid shitting on Tom's car? Tom's gonna go off on one in a minute.
by Manson April 27, 2005
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