(1)-v. The act of creating, participating in, or spreading a conspiracy theory, especially one which would be found in conservative/right-wing circles;
(2)-adj. Akin to a conspiracy of ridiculous proportions, as would be put forward by the John Birch Society (JBS), Robert Welch's far-right anti-communist society.
Jill: "Did you catch Robert's grandmother yesterday? She was talking like Goldwater had won in '64. I guess LBJ must have hidden all those votes."
Jane: "Girl, she was just JBS-ing!"
David: "Did you hear that lecture on global warming yesterday?"
John: "Dude, global warming is so a government project...don't you know about the weather machine in DC?"
David: "That is so JBS-ing"