Originally from Rocky Horror Picture Show, the phrase is now used when someone is about to tell you a story you don't want to hear, in the hopes that they will be confused/distracted enough for you to change the subject or flee the scene.
Exercise caution when the mark is actually named Janet.
Talia: That reminds me of when I had a four hour conversation with Ben Stein about economic theory...
Bruce: Tell us about it, Janet!
Talia: Wh-
Bruce: *flees*
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