noun. Apparently, an asshole who rides their bike on the sidewalk. Or... a bad, bad muppet.
Real citation: "Yeah, well I don't want to be 'sidewalk-kill' either, so get your bike the hell off the sidewalk. Just today I almost got nailed TWICE by jerkoid bicyclists, and one of them even swore at ME for not watching where I was going! Hey pal, I don't have eyes in the back of my head and neither does anyone else, and in any case I shouldn't have to be on the lookout for bicyclists coming up behind me or beside me. So if you absolutely must be a jagmuppet and ride on the sidewalk where you're not supposed to be, at least warn the people in front of you that you're approaching."
(Sept. 12, 2007, City-Data Forum, http://www.city-data.com/forum/chicago/153136-sidewalk-bicycling-chicago.html)
Made-up citation: "To people of my advanced age, Elmo will always be a rancid little jagmuppet. I knew Cookie Monster, and you are no Cookie Monster."