1: A disinclination to think imaginatively.
2: A block to one's imagination and originality caused by lack of inspiration; an inert imagination.
Imaginertia occurs when one's imagination becomes inert, e.g. it stops working. The scientific explanation is that there is no true originality. All creative thoughts are comprised of what the mind already knows and how it reorganises that knowledge to create an entirely new thought (otherwise known as using one's imagination). When lacking an inspiring source one is prone to imaginertia, but it can pass as quickly as it comes.
Despite previous signs of originality, the comedian fell flat. The audience saw his imaginertia and sighed.