First, a negatable hypothesis is one that can be proven wrong. For example, if I hypothesize that nitric acid and glycerin make nytro-glycerin, you can check the validity by mixing the 2 chemicals together and seeing if you blow up. (you will as this is an exothermic reaction...)

So, a non-negatable hypothesis is one that can NOT be proven wrong. For example, if I claim that invisible, undetectable, purple unicorns in the constellation of Orion control the fate of the universe, you can not prove me wrong. Or, if I claimed that a space craft in the tail of a comet would transport us to another dimension, if we committed suicide (as the Heaven's Gate people believed) you could not prove me wrong. (Too bad they did not know about the principle of a non-negatable hypothesis!)

A non-negatable hypothesis is invalid.