2)to have a GDRITBY: to be bored,
3)to build a GDRITBY, to kill time,
4)to be a GDRITBY, to be an amusing waste of time, an easy ride, or a complete wank.
(it is considered a faux-pass to specify possesion of the backyard in question)
Of Washington D.C./VA/MD (USA) regional origin, generaly thought to have been inspired by an episode of Myth-Busters, and; or the ramblings of a random meth-head who works in a drivethru.
"Not much, just building a giant death-ray in the backyard."