An unknown existential risk is an existential risk which has not yet been identified.
Even at the point when Leo Szilard discovered the concept of a nuclear chain reaction, other nuclear scientists were dismissive of the possibility of ever generating energy from atomic forces. The first atomic bombs were built just over a decade later.
In the same way, ideas which we currently laugh off might one day become a reality. Such scenarios are extremely hard to predict and plan around. Most of the ideas we currently laugh off will probably continue to be laughable. Yet it would be overconfident to assume that we have already identified all significant existential risks, or that we are at present in a position to do so (Bostrom 2002, sect. 4.7).
Bostrom, Nick (2002) Existential risks: Analyzing human extinction scenarios and related hazards, Journal of Evolution and Technology, vol. 9.