(noun) the resulting effect of derp spreading rapidly between individuals in a group, as a result of hyperstimulation of the derp gland. a single stupid person will be stimulated by something derp-worthy, which will cause them to emanate derp out to their surroundings. other stupid individuals will receive the stimulus and produce the same effect. this continues until an entire population of similarly-minded individuals have become stimulated, which can lead to widespread epic failure. it becomes increasingly more common in societies where intelligence is not respected (e.g. America)
Twilight. I don't need to say anything else. Fucking Twilight. Stephenie Meyer originated the derp signal, which spread like wildfire to its readers, resulting in a derp cascade. There are plenty more examples, but for now, this epitomizes it.