"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
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 or Kopta Rokkits. 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, or if it is in fact a gyrocopter in which case some form of forward-thrusting engine is completely necessary, which just goes to show how much the writers here know about real life hardware). 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 or even their own rotor blades to mulch men into tomatoe juice, 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, if not in 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.
On the Tabletop
On the tabletop Deffkoptas can be a highly effective asset, despite being rather pricey points-wise. While they are fragile they are VERY fast, and by using their speed and Flier mobility in conjunction with the Scoutin' Ahead rule you can apply their respectable firepower to wherever it is most needed. In a pinch they can also use their Spinnin' Blades and Sluggas to perform as a semi-decent melee unit. That being said, due to their small unit size and vulnerability to casualties, Deffkoptas should engage in melee only when absolutely necessary to finish off a highly depleted target.
Unfortunately, Deffkoptas are perhaps not as common in Ork armies as they should be, because the only model currently available through GW is a piece of crap pewter dual-rotor revolver-armed model. Perhaps you've been playing 40k for a while, and if so, you may remember how your old pewter minis would always fall apart every time you looked at them. Well the pewter Deffkopta remembers, and if just looking at this thing doesn't cause you to experience hobbyist PTSD, then you might be short a few marbles. Anyway, the pewter Deffkopta is hands-down one of the worst models in the whole range and you should not buy this poorly-designed, fiddly, old-ass miniature under any circumstance.
What makes the current situation with Deffkoptas even worse is that in 5th edition there were actually decent plastic models available for them, which were included in the starter set Assault On Black Reach (a converted example is shown in the picture above). Unfortunately, these plastic Deffkoptas came as an integral part of a combined Ork sprue that is now discontinued, and as such they can no longer be purchased from Games Workshop directly. However, they can still be obtained singly through the "Battle for Vedros" and the "Build + Paint" series of miniature kits; these kits were distributed through alternative outlets such as toy stores rather than hobby stores, and as of early 2020 many places still have them in stock. That being said, your best bet for obtaining plastic Deffkoptas these days, especially if you want a group of them, is probably through Ebay. In the meantime, hopefully a future Orktober will grace us with a new plastic Deffkopta kit.
GOOD NEWS EVERYONE! The Deffkopta is back in plastic form for 2021! The new Deffkoptas are very similar in form to the ones from Black Reach. They also regain the Big Bomm and Kustom Mega-Blasta options, but sadly they can no longer be equipped with Big Shootas (which has gotta be damn annoying to anyone who took the time to equip their Koptas this way).
A pewter Deffkopta. Believe it or not, this model actually depicts a Gorkamorka special character named Dregmek Blitzkart, which should give you an idea of just how hilariously old it is. GW, please, just end its misery.