A term used to describe any out-dated or archaic technology especially one that is inherently dumb.
Before the Emti and Gravity Drive were developed the only reliable way of getting anything off the Earth was by using a rocket, which essentially creates forward movement by chucking something out the back, usually at very high speeds. During what was laughingly described as The Space Age unbelievable resources were literally thrown away.
While it is difficult to believe, it is actually true that explosive chemicals were packed into metal cylinders, and essentially set light to. More amazingly, human beings volunteered to sit on top of these guided bombs in order to spend a very short time in a low orbit around the Earth. It says a lot about the human spirit and very little about human sanity.
Not only were these devices dangerous and impractical they were gloriously inefficient. All the chemical propellant (the stuff they threw out the back) had to be carried at launch so a rocket that was designed to carry a payload into Earth orbit was unbelievably heavy. The heavier the payload, the more rocket fuel was required, which of course added to the weight and therefore needed more fuel to lift the fuel. It was common that 90% of the weight of a launch vehicle was actually the fuel to lift it! We're not making this up! Honest! 
The phrase "it's not rocket science" therefore means something that is contemporary and efficient. To describe someone as a 'rocket scientist' is likely to result in verbal or even physical violence.