"Jumping the shark" means that they've run out of ideas, done something really stupid, and there's no turning back--it's pretty much all over but the cancellation. The term is taken from the Happy Days episode where Fonzie literally jumped a shark on water skis. Although the series continued for several years, it was never the same. It had crossed the line into complete absurdity: the point of no return.
I don't know what season it was, but the VOY episode that had Janeway, Tuvok, Torres "partially assimilated" then "un-assimilated" was a shark jump if I've ever seen one.
"Are you out of your Vulcan mind?"