"Wot's faster than a warbuggy, more killy than a warbike, and flies through da air like a bird? I got no bleedin' idea, but I'm gonna find out."
- – Kog da Flymek, pioneer of the Deffkopta
"Harharhar! Humie don' surf!"
No, but seriously
A Deffkopta is an Ork flying device resembling a crude helicopter or gyrocopter built to seat one (the pilot) and carry a set of underslung weaponry, generally twin-linked big shootas, bazookas or mega-blastas. The vehicle usually has additional rocket thrusters installed on it to aid the velocity-addicted greenskin manning it to get his nonexistent rocks off even harder than already possible, notwithstanding how senseless jamming rockets onto a helicoper is (unless they're the shoot-at-the-other-guy kind). Some daring flyboys go so far as to modify their rawket lawnchair with a chainsaw or rotary saw blade sticking out the bottom to driveby-slit-up gits with, unheeding of the danger of snagging it on something and then slamming face first into the ground. It is the logical extension of the average Kult of Speed fanatics' MO and one would not at all be wrong to think of it as essentially being an airborne Waaaghbike, sometimes even being constructed from the remains of one (though that doesn't mean much with Orks anyway).
So it came to pass that in the 41st millenium, as the humans were rocking a transport craft that was the Mil Mi-24 Hind in design, carrying capacity, combat role, and armament but exactly not name, the Orks were still the only ones with any actual helicopters. Well, sort of. Gyrocopters of one kind or another make several appearances in the literature, but these are almost exclusively for civilian and non-combat roles, so rather, the Orks are the only ones that still fly helicopters into battle.
It should be noted that you are extremely unlikely to face one in an actual battle though, as the only model available for sale is a piece of crap pewter dual-rotor revolver-armed model. In 5th edition, there were actually alright plastic models for the Deffkopta, which appeared in the starter set Assault On Black Reach. But now the only way to get your grubby little mitts on the things is Ebay, or other third parties, as to my knowledge they never saw a separate release, and even if they did they no longer have it. So until GeeDubs get around to actually giving these guys viable models don't expect to lock horns with this beast any time soon. (A few were released in plastic for the Assault on Black Reach starter set, which would resurface, albeit in colored plastic, for the Battle for Vedros starter sets.)