Originating from the idea of ordering the most obnoxious drink at the bar, whipwacker can be likened to an obnoxious person or group - specifically used for bachelorette parties in Nashville.
What happens in online discussion when a bunch of Doctor Who nerds hijack the thread to discuss the ins and outs of that 50-year-old show, generally losing half the audience.
I posted in Facebook asking for some good shows to watch and 30 comments later, the discussion had been Whojacked by a bunch of peopletalking about Matt Smith and David Tennant.